UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


S266  •  1  1 


Marie  pinx' 


HALO 


THE  ATMOSPHERE. 


TRANSLATED  FROM  THE  FRENCH 


CAMILLE  FLAMMARION, 


EDITED  BY 

JAMES   GLAISHER,  F.R.S., 

SUPERINTENDENT  OF  THE   MAGNETICAL  AND   METEOROLOGICAL   DEPARTMENT 
OF  THE  ROYAL  OBSERVATORY  AT  GREENWICH. 


WITH  TEN  CHROMO-LITHOGRAPHS  AND  EIGHTY  -  SIX  WOODCUTS 


NEW    YORK: 
HARPER     &     BROTHERS,    PUBLISHERS, 

FRANKLIN     SQUARE. 
1874. 


PREFACE  BY  THE  EDITOR. 


THE  following  work  is  translated  and  abridged  from  M.  Flammarion's 
LJ 'Atmosphere,  Paris,  1872.  That  some  curtailment  of  the  text  of  the 
original  work  was  requisite  will  be  apparent  when  it  is  stated  that  the 
French  Edition  contains  824  large  pages  of  closely  printed  matter,  and 
is  of  more  than  twice  the  extent  of  the  present  volume.  Not  only  was 
some  compression  necessary  in  order  to  bring  the  work  within  a  rea- 
sonable compass,  but,  independently  of  this,  one  or  two  chapters,  such 
as  that  on  the  Eespiration  and  Alimentation  of  Plants,  appeared  to  have 
so  remote  a  connection  with  the  subject  of  the  work — the  Atmosphere 
— that  their  omission  would  in  any  case  have  been  desirable. 

Every  one  who  has  any  acquaintance  with  French  popular  works  on 
Science  is  aware  that  very  many  exhibit  a  tendency  to  imaginative,  or, 
to  express  my  meaning  colloquially,  "fine"  writing,  which  ill  accords 
with  the  precision  and  accuracy  that  ought  to  be  a  characteristic  of 
scientific  information,  even  when  expressed  in  language  free  from  tech- 
nicalities. There  is  a  good  deal  of  this  exalted  kind  of  composition  in 
M.  Flammarion's  book,  which — even  in  the  French  not  very  agreeable 
to  an  English  reader — becomes,  when  translated,  intolerable.  I  have, 
therefore,  omitted  these  rhapsodies  very  freely,  though  traces  enough 
of  them  will  be  found  here  and  there  to  betray  the  French  origin  of 
the  work. 

I  may  add  that  the  task  of  editing  has  not  been  a  light  one ;  besides 
the  necessity  for  compression  and  the  consequent  selection  of  the  mat- 
ter to  be  included,  I  have  been  obliged  to  exercise  some  sort  of  censor- 
ship over  the  facts  contained  in  the  work.  It  is  impossible  for  any  one 
man  to  have  a  complete  knowledge  of  so  great  a  variety  of  subjects  as 
are  treated  of  by  M.  Flammarion,  and  the  compiler  of  such  a  book  must 
include  many  things  taken  from  others,  of  the  accuracy  of  which  he  is 
not  fully  competent  to  judge.  In  cases  where  a  statement  contained  in 
the  original  work  appeared  to  me  clearly  erroneous,  I  have  corrected  it, 


445073 


4  PREFACE  BY  THE  EDITOR. 

appended  a  note,  or  omitted  it  altogether ;  and  in  cases  where  I  have 
been  doubtful  of  the  accuracy  of  a  passage,  or  have  differed  in  opinion 
from  the  author,  I  have  not  considered  myself  justified  in  making  an 
alteration,  so  long  as  there  was  no  strong  primd  facie  presumption  that 
the  original  was  incorrect.  In  spite  of  obvious  blemishes,  inseparable 
from  a  translation,  and  a  certain  want  of  continuity  in  a  few  places, 
which  is  due  to  the  omission  of  portions  of  the  book  as  originally 
written,  I  believe  the  volume  will  be  found  to  be  readable,  popular,  and 
accurate,  and  it  covers  ground  not  occupied  by  any  one  work  in  our 
language. 

The  work  treats  on  the  form,  dimensions,  and  movements  of  the 
earth,  and  of  the  influence  exerted  on  meteorology  by  the  physical 
conformation  of  our  globe;  of  the  figure,  height,  color,  weight,  and 
chemical  components  of  the  atmosphere;  of  the  meteorological  phe- 
nomena induced  by  the  action  of  light,  and  the  optical  appearances 
which  objects  present  as  seen  through  different  atmospheric  strata ;  of 
the  phenomena  connected  with  heat,  wind,  clouds,  rain,  and  electricity, 
including  the  subjects  of  the  laws  of  climate:  the  contents  are  there- 
fore of  deep  importance  to  all  classes  of  persons,  especially  to  the  ob- 
server of  nature,  the  agriculturist,  and  the  navigator. 

The  whole  is  explained  in  a  very  popular  manner,  and  as  free  as  pos- 
sible from  all  technicalities ;  the  object  having  been  to  produce  a  work 
giving  a  broad  outline  of  the  causes  which  give  rise  to  facts  of  every- 
day occurrence  in  the  atmosphere,  in  such  a  form  that  any  reader  who 
wished  to  obtain  a  general  view  of  such  phenomena  and  their  origin 
would  be  readily  enabled  to  do  so.  The  great  number  of  subjects 
treated  of  will  thus,  to  the  majority  of  readers,  who  merely  desire  an 
insight  into  the  general  principles  that  produce  phenomena  which 
every  one  has  seen  or  heard  of,  be  found  to  be  rather  an  advantage,  as 
the  whole  range  of  atmospheric  action  is  thus  displayed  in  the  same 
volume  in  moderate  compass,  without  so  much  detail  being  anywhere 
given  as  to  make  the  book  other  than  interesting  to  even  the  most 
casual  reader. 

The  translation  was  made  by  Mr.  C.  B.  PITMAN. 

January,  1873. 


PREFACE. 


In  esi  vivimus,  movemur  et  sumus. 

OF  all  the  various  subjects  which  invite  a  studious  examination,  it  is 
impossible  to  select  one  possessing  a  more  direct,  a  more  permanent,  or 
a  more  real  interest  than  that  which  forms  the  subject  of  this  work. 
The  Atmosphere  gives  life  to  earth,  ocean,  lakes,  rivers,  streams,  forests, 
plants,  animals,  and  men ;  in  and  by  the  Atmosphere  every  thing  has 
its  being.  It  is  an  ethereal  sea  reaching  over  the  whole  world;  its 
waves  wash  the  mountains  and  the  valleys,  and  we  live  beneath  it  and 
are  penetrated  by  it.  It  is  the  Atmosphere  which  makes  its  way  as  a 
life-giving  fluid  into  our  lungs,  which  gives  an  impulse  to  the  frail  ex- 
istence of  the  new-born  babe,  and  receives  the  last  gasp  of  the  dying 
man  upon  his  bed  of  pain.  It  is  the  Atmosphere  which  imparts  verd- 
ure to  the  fertile  fields,  nourishing  at  once  the  tiny  flower  and  the 
mighty  tree ;  which  stores  up  the  solar  rays  in  order  to  give  us  the 
benefit  of  them  in  the  future.  It  is  the  Atmosphere  which  adorns  with 
an  azure  vault  the  planet  in  which  we  move,  and  makes  us  an  abode  in 
the  midst  of  which  we  act  as  if  we  were  the  sole  tenants  of  the  infinite 
— the  masters  of  the  universe.  It  is  the  Atmosphere  which  illuminates 
this  vault  with  the  soft  glitter  of  twilight,  with  the  waving  splendors  of 
the  aurora  borealis,  with  the  quivering  of  the  lightning  and  the  multi- 
form phenomena  of  the  heavens.  At  one  moment  it  inundates  us  with 
light  and  warmth,  at  another  it  causes  the  rain  to  pour  down  in  torrents 
upon  the  thirsty  land.  It  is  the  channel  by  which  the  sweet  perfumes 
descend  from  the  hills,  and  the  vehicle  of  the  sound  which  permits 
human  beings  to  communicate  with  each  other,  of  the  song  of  the  birds, 
of  the  sighing  of  the  wind  among  the  trees,  of  the  moaning  of  the  waves. 
Without  it,  our  planet  would  be  inert  and  arid,  silent  and  lifeless.  By 
it  the  globe  is  peopled  with  inhabitants  of  every  kind.  Its  indestructi- 
ble atoms  incorporate  themselves  in  the  various  living  organisms;  the 


Q  PREFACE. 

particle  which  escapes  with  our  breath  takes  refuge  in  a  plant,  and, 
after  a  long  journey,  returns  to  other  human  bodies ;  that  which  we 
breathe,  eat,  and  drink  has  already  been  inhaled,  eaten,  and  drunk  mill- 
ions of  times :  dead  and  living,  we  are  all  formed  of  the  same  sub- 
stances. ....  What  study  can  possess  a  vaster  or  more  direct  interest 
than  that  of  the  vital  fluid  to  which  we  owe  the  manner  of  our  being 
and  the  maintenance  qf  our  life? 

The  study  of  the  Atmosphere,  of  its  physical  condition,  of  its  move- 
ments, of  its  functions,  and  of  the  laws  which  regulate  its  phenomena, 
forms  a  special  branch  of  human  research.  This  science,  which  since 
the  days  of  Aristotle  has  been  designated  Meteorology,  belongs  in  part 
to  Astronomy,  which  shows  the  movements  of  our  planet  around  the 
sun — movements  to  which  we  owe  day  and  night,  season,  climates,  solar 
action,  or,  in  a  word,  the  basis  of  the  subject.  On  the  other  hand,  it 
appertains  to  Natural  Philosophy  and  Mechanics,  which  explain  and 
measure  the  forces  brought  into  play.  As  it  exists  in  the  present  day, 
Meteorology  is  a  new  science,  of  recent  establishment,  scarcely  as  yet 
fixed  in  its  elementary  principles. 

We  are  assisting  at  its  elaboration,  at  its  struggling  into  life.  The 
present  generation  has  seen  the  establishment  of  meteorological  societies 
throughout  the  different  nations  of  Europe,  and  of  sp.ecial  observatories 
for  the  exclusive  study  of  the  problems  relating  to  the  Atmosphere. 
The  analysis  of  climates,  seasons,  currents,  and  periodical  phenomena 
is  scarcely  terminated.  The  examination  of  atmospheric  disturbances, 
of  tempestuous  movements,  and  of  storms,  has  been  made,  so  to  speak, 
before  our  own  eyes.  The  science  of  the  Atmosphere  is  the  question 
of  the  day.  We  are  just  now,  in  regard  to  this  study,  in  an  analogous 
situation  to  that  of  modern  Astronomy  in  the  days  of  Kepler.  Astron- 
omy was  founded  in  the  seventeenth  century.  Meteorology  will  be  the 
work  of  the  nineteenth. 

I  have  endeavored  to  collect  in  this  work  all  that  is  at  present  posi- 
tively known  about  this  important  subject,  to  represent  as  completely 
as  possible  the  actual  state  of  our  knowledge  about  the  Atmosphere 
and  its  work — that  is,  about  the  air,  the  seasons,  the  climates,  the  winds, 
the  clouds,  the  rain,  the  hurricanes,  the  storms,  the  lightning,  the  me- 
teors—in a  word,  the  phenomena  of  time,  and  above  all,  the  general 
upholding  of  terrestrial  life.  It  is,  in  fact,  a  synthesis  of  the  research 
effected  during  the  last  half  century  (especially  during  the  latter  portion 
of  it)  as  to  the  great  phenomena  of  terrestrial  nature,  and  the  forces 


PREFACE.  7 

.which  produce  them.  The  great  majority  of  us,  inhabitants  of  the 
earth,  no  matter  to  what  nation  we  belong,  pass  our  lives  without  at- 
tempting to  form  an  idea  of  our  actual  position,  without  asking  ourselves 
what  is  the  force  which  prepares  for  us  our  daily  bread,  ripens  for  us 
the  grapes  that  give  the  wine,  presides  over  the  change  in  the  seasons, 
and  alternates  the  exhilarating  blue  sky  with  the  rains  and  cold  of  in- 
hospitable winter.  Yet,  why  should  we  live  in  such  a  state  of  igno- 
rance? I  venture  to  hope  that  after  perusing  this  work  there  will  be 
no  difficulty  in  understanding  the  life  and  movements  of  the  globe. 
Every  thing  which  takes  place  around  us  is  interesting  when,  instead  of 
remaining  as  one  born  blind,  a  man  has  learned  to  appreciate  external 
things  and  to  keep  himself  in  intelligent  communication  with  Nature. 

I  could  have  wished  to  keep  this  work,  destined  as  it  is  for  the  gen- 
eral public,  free  from  scientific  terms  and  figures  which  constitute  its 
basis.  I  have  done  so  as  far  as  possible,  but  without  in  any  point  sacri- 
ficing accuracy  and  precision  in  respect  to  observed  facts.  It  seems  to. 
me,  too,  that  what  is  termed  the  public  (that  is,  every  one)  has  become 
somewhat  scientific  itself,  since  so  many  excellent  works  have  popular- 
ized ideas  previously  reserved  for  a  small  circle  of  the  elect. 

CAMILLE  FLAMMARION. 

PARIS  :  November,  1871. 


CONTENTS. 


BOOK  FIRST. 

OUR  PLANET  AND  ITS  VITAL  FLUID. 

CHAP.  PAGE 

I.  THE  TERRESTRIAL  GLOBE 17 

II.  THE  ATMOSPHERIC  ENVELOPE 23 

III.  THE  HEIGHT  OF  THE  ATMOSPHERE 28 

IY.  WEIGHT  OF  THE  TERRESTRIAL  ATMOSPHERE — THE  BAROMETER 

AND  ATMOSPHERIC  PRESSURE  ......    38 

V.  CHEMICAL  COMPONENTS  OF  THE  AIR 57 

VI.  SOUND  AND  THE  VOICE 75 

VII.  AERONAUTICAL  ASCENTS  . .  85 


BOOK  SECOND. 

LIGHT  AND  THE  OPTICAL  PHENOMENA  OF  THE  AIR. 

I.  THE  DAY 103 

II.  EVENING 113 

III.  THE  RAINBOW 121 

IV.  ANTHELIA  :   SPECTRE-SHADOWS  UPON  MOUNTAINS — THE  ULLOA 

CIRCLE — CIRCLE  SEEN  FROM  A  BALLOON 127 

V.  HALOS  :    PARHELIA  —  PARASELENES  —  CIRCLES    SURROUNDING 

AND    TRAVERSING    THE     SuN — CORONAS  —  COLUMNS — VARI- 
OUS PHENOMENA 137 

VI.  THE  MIRAGE..  ...   H9 


IQ  CONTENTS. 

CHAP.  PAGE 

VII  SHOOTING-STARS — BOLIDES — AEROLITES — STONES  FALLING  FROM 

THE  SKY 163 

VIIL  THE  ZODIACAL  LIGHT 174 


BOOK  THIRD. 

TEMPERATURE. 

I.  HEAT:   THE  THERMOMETER — QUANTITY  OF  HEAT  RECEIVED  — 

TEMPERATURE  OF  THE  SUN — TEMPERATURE  OF  SPACE 181 

II.  HEAT  IN  THE  ATMOSPHERE 190 

III.  THE  TEMPERATURE  OF  THE  AIR  :  ITS  MEAN  CONDITION — DAILY 

AND  MONTHLY  VARIATIONS  OF  THE  TEMPERATURE  —  TEM- 
PERATURE OF  EACH  SUMMER,  WINTER,  AND  YEAR  AT  PARIS 
AND  AT  GREENWICH  SINCE  THE  LAST  CENTURY — DAILY  AND 
MONTHLY  VARIATIONS  OF  THE  BAROMETER 202 

IV.  REMARKABLE  SUMMERS— THE  HIGHEST  KNOWN  TEMPERATURES  218 

V.  AUTUMN — WINTER  :  WINTER  LANDSCAPES — COLD — SNOW — ICE 
— HOAR-FROST,  RIME,  ETC.  —  REMARKABLE  WINTERS — THE 
LOWEST  KNOWN  TEMPERATURES 229 

VI.  CLIMATE  :  DISTRIBUTION  OF  TEMPERATURE  OVER  THE  GLOBE — 
ISOTHERMAL  LINES — THE  EQUATOR  —  THE  TROPICS — THE 
TEMPERATE  REGIONS  —  THE  POLES  —  THE  CLIMATE  OF 
FRANCE .  245 


BOOK  FOURTH. 

THE   WIND. 

I.  THE  WIND  AND  ITS  CAUSES:  GENERAL  CIRCULATION  OF  THE 
ATMOSPHERE  —  THE  REGULAR  AND  PERIODICAL  WINDS  — 
TRADE-WINDS — THE  MONSOON — BREEZES 269 

II.  THE  SEA  CURRENTS  :  METEOROLOGY  OF  THE  OCEAN— MARITIME 

ROUTES — THE  GULF  STREAM 284 


CONTENTS.  11 

CHAP.  PAGE 

III.  THE  VARIABLE  WINDS — THE  WIND  IN  OUR  CLIMATES — MEAN 

DIRECTIONS  IN  EUROPE  AND  IN  FRANCE  —  RELATIVE  FRE- 
QUENCY OF  DIFFERENT  WlNDS — RlSE  OF  THE  WlNDS  ACCORD- 
ING TO  THE  TlMES  AND  PLACES MONTHLY  AND  DlURNAL  VA- 
RIATION IN  INTENSITY . .  297 

IV.  RESPECTING  CERTAIN  SPECIAL  WINDS  :  THE  BISE — THE  BORA — 

THE  GALLEGO — THE  MISTRAL — THE  HARMATTAN — THE  SI- 
MOOM— THE  KHAMSEEN — THE  SIROCCO — THE  SOLANO 318 

V.  THE  POWER  OF  THE  AIR:   THE  HURRICANE — THE  CYCLONE — 

THE  TEMPEST 327 

VI.  TROMBES,  WHIRLWINDS,  OR  WATER-SPOUTS 337 


BOOK  FIFTH. 

WATER— CLO  UDS—RAIN. 

I.  THE  WATER  UPON  THE  SURFACE  OF  THE  EARTH  AND  IN  THE  AT- 
MOSPHERE :  THE  EARTH — VOLUME  AND  WEIGHT  OF  THE  WA- 
TER THROUGHOUT  THE  GLOBE PERPETUAL  CIRCULATION 

VAPOR  OF  WATER  IN  THE  ATMOSPHERE — ITS  VARIATIONS  AC- 
CORDING TO  THE  HEIGHT,  THE  LOCALITY,  AND  THE  WEATHER 
— THE  HYGROMETER — DEW — WHITE  FROST 355 

II.  THE  CLOUDS  :  WHAT  A  CLOUD  is — THE  MANNER  OF  ITS  FORMA- 
TION— MIST — OBSERVATIONS  TAKEN  FROM  A  BALLOON  AND 
FROM  MOUNTAINS  —  DIFFERENT  KINDS  OF  CLOUDS  —  THEIR 
SHAPES— THEIR  HEIGHTS 363 

III.  RAIN:  GENERAL  CONDITIONS  OF  THE  FORMATION  OF  RAIN — ITS 

DISTRIBUTION  OVER  THE  GLOBE — RAIN  IN  EUROPE 381 

IV.  HAIL:   PRODUCTION  OF  HAIL — COURSE  OF  HAILSTORMS — VARY- 

ING DISTRIBUTION  OF  HAILSTORMS  IN  DIFFERENT  PARTS  OF 
THE  COUNTRY  —  HEAVIEST  HAILSTORMS  KNOWN  —  NATURE, 
SIZE,  AND  SHAPE  OF  HAILSTONES — PERIODS  OF  THEIR  OCCUR- 
RENCE    390 

V.  PRODIGIES:  SHOWERS  OF  BLOOD  —  OF  EARTH  —  OF  SULPHUR  — 
OF  PLANTS — OF  FROGS — OF  FISH — OF  VARIOUS  KINDS  OF  AN- 
IMALS.. .  401 


12  CONTENTS. 

BOOK  SIXTH. 

ELECTRICITY,  THUNDER-STORMS,  AND  LIGHTNING. 

CHAP.  PAGE 

I.  ELECTRICITY  UPON  THE  EAKTH  AND  IN  THE  ATMOSPHERE  :  ELEC- 
TRIC CONDITION  OF  THE  TERRESTRIAL  GLOBE — DISCOVERY  OF 
ATMOSPHERIC  ELECTRICITY — EXPERIMENTS  OF  OTTO  DE  GUE- 
RICKE,  WALL,  NOLLET,  FRANKLIN,  ROMAS,  RICHMANN,  SAUS- 
SURE,  ETC. — ELECTRICITY  OF  THE  SOIL,  OF  THE  CLOUDS,  OF  THE 
AIR — FORMATION  OF  THUNDER-STORMS.  , 423 

IL  LIGHTNING  AND  THUNDER 431 

III.  THE  SAINT  ELMO  FIRES  AND  THE  JACK-O'-LANTERNS 441 

IV.  AURORA  BOREALES .445 


ILLUSTRATIONS. 


CHR  OMO-LITHO  GRAPHS. 

FIG.  PAGE 

1.  Halo Frontispiece. 

2.  Sunset  at  Sea Toface  119 

3.  The  Rainbow "       121 

4.  Lunar  Rainbow  seen  at  Compiegne "       126 

5.  Sunrise  from  the  Righi "       127 

149 
218 


6.  African  Mirage 

7.  Summer  Landscape 

8.  Winter  Landscape 

9.  The  Storm 

10.  Aurora  Borealis  seen  at  Paris,  May  13,  1869. 


423 
445 


WOOD-CUTS. 

1.  Mathematical  Limit  of  the  Shape  of  the  Atmosphere 29 

2.  Measure  of  the  Height  of  the  Atmosphere,  according  to  the  Length  of  Twilight 32 

3.  Thickness  of  the  Earth's  Crust,  of  our  Atmosphere,  and  of  a  higher  Atmosphere 34 

4.  Suction-Pump 39 

5.  Suction  and  Forcing  Pump 40 

6.  Torricelli  inventing  the  Barometer 41 

7.  Barometer  Tube  full  of  Quicksilver 43 

8.  The  Tube  in  the  Basin 43 

9.  Otto  de  Guericke's  Experiment 45 

10.  The  Magdeburg  Hemispheres .' .'  46 

11.  Atmospheric  Pressure.     Rupture  of  Equilibrium 47 

12.  Atmospheric  Pressure  under  an  inverted  Glass 47 

13.  Diagram  showing  the  Decrease  of  atmospheric  Pressure,  according  to  Height 51 

14.  Variation  in  the  atmospheric  Pressure  at  the  Level  of  the  Sea 52 

15.  Lavoisier  analyzing  atmospheric  Air 56 

16.  Matrass  or  Glass  Vessel 58 

17.  The  Apparatus  for  Analysis  of  Air 58 

18.  Mercury-Eudiometer,  for  analyzing  Air 59 

19.  Apparatus  for  analyzing  Air  by  the  Method  of  Weight 60 

20.  Apparatus  for  obtaining  the  Proportion  of  carbonic  Acid  in  Air 61 

21.  Apparatus  for  separating  the  Oxygen  from  the  Nitrogen 62 

22.  Vibrations  of  a  Blade 75 

23.  Vibration  of  a  Cord 76 

24.  Illustration  of  Hawksbee's  Experiment 78 

25.  Baroscope 86 

26.  Soap-bubbles  inflated  with  Hydrogen 88 

27.  Distribution  of  Kinds  of  Birds  according  to  Height  of  Flight 97 

28.  Lunar  Day Ill 

29.  Atmospheric  Refraction 114 

30.  Simple  Reflection  of  Rays  in  a  Drop  of  Rain 121 


14  LIST  OF  ILLUSTRATIONS. 

PAGE 
FIO-  -,  9q 

31.  Formation  of  the  Rainbow 

32.  Double  Keflection  of  Rays  in  a  Drop  of  Rain 1^ 

33.  Theory  of  the  two  Arches  of  a  Rainbow 12* 

34.  Triple  Rainbow 12 

35.  The  Spectre  of  the  Brocken 12 

36.  The  Ulloa  Circle 132 

37.  Theory  of  the  Halo u 

38.  Halo  seen  in  Norway • ^ 

39.  Corona  formed  around  the  Moon  by  Diffraction 147 

40.  Explanation  of  the  ordinary  Mirage 152 

41.  Mirage  seen  at  Paris  in  1869 158 

42.  Lateral  Mirage  seen  on  the  Lake  of  Geneva 160 

43.  La  Fata  Morgana I62 

44.  Shooting-stars 165 

45.  Fall  of  a  Bolide  in  the  Daytime 170 

46.  The  Caille  Aerolite,  weighing  12^  cwt 172 

47.  The  Pyrheliometer 183 

48.  Relative  Intensity  of  the  calorific,  luminous,  and  chemical  Rays  of  the  Sun 192 

49.  Inequality  of  the  Thickness  of  Air  traversed  by  the  Sun 196 

50.  Regular  Diurnal  Oscillation  of  the  Barometer 213 

51.  Regular  Monthly  Oscillation  of  the  Barometer 216 

52.  Snow  Crystals 231 

53.  Winter.— The  Seine  full  of  floating  Ice 235 

54.  Comparative  Temperatures  of  Rome,  London,  Paris,  Vienna,  St.  Petersburg 251 

55.  The  last  human  Dwelling-places.     Esquimaux  of  the  Polar  Regions 262 

56.  Ice  at  the  Pole 264 

57.  Section  of  the  Atmosphere,  showing  its  general  Circulation 272 

58.  Average  annual  Prevalence  of  different  Winds  at  London 305 

59.  Average  annual  Prevalence  of  the  different  Winds  at  Brussels 305 

60.  Monthly  Intensity  of  the  Winds 307 

61.  Diurnal  Intensity  of  the  Winds 307 

62.  The  Simoom 323 

63.  Whirlwind 346 

64.  Sand  Whirlwind 348 

65.  Water-spout  at  Sea 350 

66.  Intense  Fog  in  one  of  the  Islands  of  the  Antipodes 368 

67.  Intense  Fog  in  the  Spitzbergen  Mountains 369 

68.  Formation  of  a  Thunder-cloud 377 

69.  Above  and  below  the  Rain-cloud 380 

70.  Diminution  in  the  Rain-fall  from  the  Tropics  to  the  Poles 383 

71.  Increase  of  Rain,  according  to  the  Undulations  of  the  Soil 384 

72.  Comparative  Depths  of  Rain-fall 385 

73.  Section  of  Hailstones,  showing  their  ordinary  interior  Structure 398 

74.  Section  of  a  Hailstone,  enlarged 399 

75.  Different  Forms  of  Hail 400 

76.  Rain  of  Blood  in  Provence,  July,  1608 404 

77.  Shower  of  Locusts 417 

78.  Shower  of  Cock-chafers 418 

79.  Experiments  of  Franklin  and  Romas 424 

80.  Richmann,  of  St.  Petersburg,  struck  by  Lightning  during  an  electrical  Experiment. .   426 

81.  Harvesters  killed  by  Lightning 438 

82.  Curious  Freak  of  Lightning * 440 

83.  Saint  Elmo  Fire  over  the  Spire  of  Notre-Dame,  Paris 442 

84.  An  Aurora  Borealis  over  the  Polar  Sea 447 

85.  Aurora  Borealis  observed  at  Bossekop  (Spitzbergen),  January  6, 1839 449 

86.  Aurora  Borealis  observed  at  Bossekop  (Spitzbergen),  January  21, 1839 451 


BOOK  FIRST. 

OUR  PLANET  AND  ITS  VITAL  FLUID. 


THE  ATMOSPHERE. 


CHAPTER  I. 

THE   TERRESTRIAL   GLOBE. 

BORNE  forward  in  space,  in  obedience  to  the  mysterious  laws  of  uni- 
versal gravity,  our  globe  travels  therein  with  a  rapidity  that  our  closest 
study  can  scarcely  conceive.  Let  us  imagine  a  sphere  absolutely  free, 
isolated  on  all  sides,  without  any  prop  or  stay,  placed  in  the  midst  of 
space.  If  this  sphere  were  alone  in  the  immensity,  it  would  remain 
thus  suspended,  motionless,  without  power  to  incline  to  this  side  or  to 
that.  Eternally  fixed,  it  would  constitute  in  itself  the  whole  of  crea- 
tion ;  astronomy  and  physics,  mechanics  and  biology,  would  all  be  in- 
cluded in  its  conception.  But  the  earth  is  not  the  only  world  existing 
in  space.  Millions  of  celestial  bodies  have  been  formed,  like  itself,  in 
the  infinite  heavens,  and  their  co-existence  establishes  between  them 
relations  inherent  in  the  very  constitution  of  matter.  The  earth,  in 
particular,  belongs  to  a  system  of  planets  analogous  to  itself,  having  the 
same  origin  and  the  same  destiny,  situated  at  various  distances  around 
the  same  centre,  and  governed  by  the  same  motive  power.  Our  planet- 
ary system  is  composed  essentially  of  eight  worlds,  made  to  revolve  in 
successive  orbits,  the  exterior  one  of  which  is  seven  thousand  million 
leagues  in  extent.  The  sun,  a  colossal  star  nearly  a  million  and  a  half 
times  larger  than  the  earth,  and  350,000  times  as  heavy,  occupies  the 
centre  of  these  orbits ;  or,  to  speak  more  accurately,  a  focus  of  one  of 
the  nearly  circular  ellipses  which  they  describe.  It  is  around  this  gi- 
gantic star  that  take  place  the  revolutions  of  the  planets,  which  are  per- 
formed with  an  indescribable  speed  on  account  of  the  length  of  the  cir- 
cumference to  be  traversed.  Far  from  being  motionless,  as  it  appears 
to  us,  the  globe  which  we  inhabit  revolves  at  an  average  distance  of 
ninety-one  and  a  half  millions  of  miles  from  the  sun,  and  over  an  orbit 


THE  ATMOSPHERE. 


which  does  not  measure  less  than  587  millions  of  miles.  These  are 
traversed  in  365  days  and  six  hours-that  is  to  say,  that  we  move 
through  space  with  a  speed  of  more  than  one  and  a  half  million  o 
miles  per  day,  or  more  than  66,000  miles  an  hour.  The  most  rapid  of 
express  trains  can  scarcely  accomplish  more  than  twenty-five  leagues 
an  hour.  Upon  the  invisible  roads  of  the  heavens  the  earth  moves 
with  a  speed  eleven  hundred  times  greater.  The  difference  is  so  enor- 
mous, that  it  is  impossible  to  express  it  in  this  work  by  a  geometrical 
figure.  If  the  distance  traversed  in  an  hour  by  a  locomotive  was  rep- 
resented by  one  tenth  of  an  inch,  it  would  be  necessary  to  trace  a  line 
more  than  nine  feet  long  to  indicate  the  comparative  advance  made  by 
our  planet  during  the  same  space  of  time.  I  will  add,  as  a  point  of 
comparison,  that  the  movement  of  the  tortoise  is  about  eleven  hundred 
times  less  rapid  than  that  of  an  express  train.  Consequently,  were  an 
express  train  to  be  sent  in  pursuit  of  the  earth,  it  would  be  as  a  tortoise 
in  pursuit  of  an  express  train. 

Situated  as  we  are  about  the  globe,  infinitely  small  mollusks,  made  to 
adhere  to  its  surface  by  its  oentral  attraction,  and  carried  away  with  it, 
we  are  unable  to  appreciate  this  movement  or  form  a  direct  idea  con- 
cerning it.  It  is  only  by  the  observation  of  the  corresponding  change 
of  position  in  the  celestial  perspectives,  and  by  calculations  based  thereon, 
that  we  have  been  able—  and  this  only  during  the  last  few  centuries— 
to  acquire  a  knowledge  of  its  nature,  its  form,  and  its  importance.  From 
the  deck  of  a  ship,  from  a  railway-carriage,  or  the  car  of  a  balloon,  we 
are  alike  unable  to  form  an  idea  of  the  movement  that  is  transferring 
us  from  one  place  -to  another,  because  we  participate  in  it  ;  and  with- 
out some  object  of  comparison  not  partaking  of  the  motion,  it  is  impos- 
sible for  us  to  appreciate  it.  To  form  an  idea  of  the  rapidity  of  the 
earth's  motion,  we  must  imagine  ourselves  placed  not  upon  the  earth's 
surface  but  outside  it,  in  space  itself,  not  far  from  the  course  along  which 
it  hurries  so  impetuously.  Then  we  should  see  far  in  the  distance  —  to 
our  left,  I  will  suppose  —  a  little  star  shining  amidst  the  rest  in  the  gloom 
of  space.  Then  this  little  star  would  seem  to  grow  larger,  and  to  draw 
nearer  to  us.  Soon  there  would  be  perceptible  a  disk  like  that  of  the 
moon,  upon  which  we  should  also  recognize  spots  formed  by  the  op- 
tical difference  between  continents  and  seas,  by  the  polar  snows  and  the 
cloudy  bands  of  the  tropics.  We  should  endeavor  to  distinguish  upon 
this  gradually  swelling  globe  the  principal  geographical  shapes  visi- 
ble athwart  the  vapors  and  clouds  of  the  atmosphere,  when  suddenly, 


THE  TERRESTRIAL   GLOBE.  19 

standing  out  against  the  sky  and  covering  the  immensity  of  its  dome, 
the  globe  would  meet  our  affrighted  gaze,  as  if  it  were  a  giant  emer- 
ging from  the  abysses  of  space.  Then,  rapidly,  without  giving  us  time 
to  recognize  it,  the  colossus  would  rush  away  to  our  right,  quickly 
diminishing  in  size,  and  silently  burying  itself  in  the  dark  depths  be- 
yond. So  moves  the  globe  we  inhabit,  and  we  are  borne  along  by  it 
like  so  many  grains  of.  dust  adhering  to  the  whirling  surface  of  a  can- 
non-ball projected  into  space. 

How  great  a  difference  there  is  between  this  truth  and  the  ancient 
fallacy  which  represented  the  earth  as  the  support  of  the  firmament! 
During  the  reign  of  illusion — so  old,  and  yet  so  difficult  to  dispel,  even 
in  our  epoch,  from  certain  minds — the  earth  was  believed  to  form  in  it- 
self alone  the  living  universe,  and  to  represent  the  whole  of  nature.  It 
was  the  centre  and  objective  of  all  creation,  while  the  rest  of  space  was 
but  a  vast  and  silent  solitude.  There  was  a  higher  region  in  the  uni 
verse — viz.,  the  heavens,  the  empyreum ;  a  lower  region — viz.,  the  earth, 
hell.  Mysticism  had  created  the  world  for  terrestrial  humanity  alone, 
as  being  the  centre  of  Divine  Will.  In  the  present  day  we  know  that 
the  heavens  are  but  boundless  space,  and  that  the  earth  is  in  the  heav- 
ens just  as  the  other  stars ;  we  contemplate  in  the  firmament  worlds  sim- 
ilar to  our  own,  and  the  starry  night  addresses  itself  to  our  minds  with 
a  new  eloquence.  The  terrestrial  globe,  with  its  humanity,  is  no  longer 
more  than  an  atom  cast  into  the  infinite — one  of  the  countless  fly-wheels 
which,  in  tens  of  thousands,  constitute  the  mysterious  mechanism  of  the 
physical  world.  Our  planetary  system,  despite  its  vastness,  compared 
to  the  microscopical  volume  of  this  earth,  is,  sun  and  all,  eclipsed  in  the 
presence  of  the  extent  and  number  of  the  stars,  which  are  solar  centres 
of  systems  distinct  from  ours.  The  astonished  gaze  encounters  distant 
suns  whose  light  takes  hundreds  and  thousands  of  years  to  reach  us,  not- 
withstanding its  wondrous  speed  of  186,000  miles  a  second ;  farther  still 
the  eye  may  contemplate  pale  masses  of  stars  which,  seen  nearer,  would 
resemble  our  Milky  Way,  and  would  be  found  to  be  composed  of  mill- 
ions of  suns  and  systems ;  beyond  these,  again,  the  eye  and  the  mind 
still  seek  to  discover  more  distant  creations,  but  the  sweep  of  our  fa- 
tigued conceptions  soon  falls  to  a  lower  level,  worn  out  and  lost  by 
this  interminable  flight  into  the  regions  of  infinity. 

An  invisible  star,  lost  in  the  myriads  of  stars,  the  earth  is  borne  along 
in  the  heavens  by  various  movements  far  more  numerous  and  peculiar 
than  most  people  would  be  inclined  to  suppose.  The  most  important 


•  THE  ATMOSPHERE. 


is  that  of  revolution,  which  we  have  noticed  above,  and  by  virtue 
£*  the  earth  moves  round  the  sun  at  the  rate  of  one  and  a  half 
miUion  of  miles  a  day.     A  second  movement,  that  of.  «te*m,  causes  it 
"  u  n  round  its  own  axis  in  the  course  of  every  four-and-twenty  hours 
t  may  be  at  once  seen,  in  examining  this  movement  of  the  globe  that 
the  different  points  of  the  terrestrial  surface  have  a  difierent  speed,  ac- 
cording to  their  distance  from  the  axis  of  rotation.     At  the  equator 
where  the  speed  is  greatest,  the  terrestrial  surface  has  to  traverse  2o,000 
miles  in  twenty-four  hours;  that  is,  more  than  1040  miles  an  hour,  or 
about  seventeen  a  minute.    In  the  latitude  of  London,  where  the  circle 
is  perceptibly  smaller,  the  speed  is  eleven  miles  a  minute.      At  Bekia- 
witz  one  of  the  towns  almost  in  the  heart  of  the  polar  region,  the  speed 
is  seven  and  a  half  miles  a  minute;  and  finally,  at  the  poles  themselves, 
it  is  nil.    A  third  movement,  that  which  constitutes  the  precession  of  the 
equinoxes,  causes  the  terrestrial  axis  to  accomplish  a  slow  rotation,  which 
occupies  not  less  than  25,868  years,  and  in  virtue  of  which  all  the  stars 
of  heaven  annually  seem  to  change  their  position,  to  return  to  the  same 
point  only  at  the  close  of  this  great  secular  cycle.     A  fourth  movement 
gradually  makes  a  change  in  the  position  of  the  perihelion,  which  makes 
the  circuit  of  the  orbit  in  20,984  years,  so  that  in  this  other  cycle  the 
seasons  successively  take  the  place  the  one  of  the  other.    A  fifth  move- 
ment causes  the  plane  of  the  earth's  orbit,  which  it  describes  around  the 
sun,  to  oscillate,  and  diminishes  the  obliquity  of  the  ecliptic  at  present, 
to  increase  it  in  the  future.     A  sixth  movement,  due  to  the  action  of 
the  moon,  and  called  nutation,  causes  the  pole  of  the  equator  to  describe 
upon  the  celestial  sphere  a  small  ellipse  in  eighteen  years  and  eight 
months.     A  seventh  movement,  caused  by  the  attraction  of  the  planets, 
and  principally  by  the  gigantic  world  of  Jupiter  and  our  neighbor  Ve- 
nus, occasions  perturbations,  calculable  beforehand,  in  the  curve  de- 
scribed by  our  planet  around  the  sun,  swelling  or  flattening  it,  according 
to  the  variations  of  distance.    An  eighth  movement,  more  considerable 
and  less  exactly  measured  than  the  preceding  ones,  though  its  existence 
is  incontestable,  is  the  transport  of  the  whole  planetary  system  in  space. 
The  sun  is  thus  not  motionless,  but  traverses  an  immense  orbital  line, 
the  direction  of  which  is  at  present  toward  the  constellation  of  Her- 
cules.    The  speed  of  this  general  movement  is  estimated  at  487,000 
miles  a  day.     The  laws  of  motion  would  incline  one  to  believe  that  the 
sun  gravitates  around  a  centre  as  yet  unknown  to  us.     If  so,  how  vast 
must  be  the  extent  of  the  circumference  of  the  ellipse  which  it  describes, 


THE  TERRESTRIAL  GLOBE.  21 

since  for  the  last  century  it  has  followed,  as  far  as  we  can  judge,  a  per- 
fectly straight  line ! 

These  different  movements,  which  cause  the  earth  to  travel  in  space, 
are  ascertained  with  certainty,  thanks  to  the  vast  number  of  the  ob- 
servations of  the  stars  made  for  more  than  4000  years,  and  to  the  defi- 
nite nature  of  the  modern  principles  of  celestial  mechanics.  The  knowl- 
edge of  these  constitutes  the  essential  basis  of  the  highest  and  most 
substantial  of  sciences.  The  earth  is  henceforth  inscribed  in  the  ranks 
of  the  stars,  in  spite  of  the  evidence  of  the  senses,  in  spite  of  secular 
illusions  and  errors,  and,  above  all,  in  spite  of  human  conceit,  which 
had  for  a  long  time  complacently  formed  a  creation  for  man  alone. 
Drawn  here  and  there  by  these  diverse  movements — some  of  which, 
such  as  that  of  the  perturbations,  are  extremely  complicated — the  terres- 
trial globe  travels  onward,  whirling  along,  balancing  itself  under  the  in- 
fluence of  varied  forces,  rushing  with  an  incomprehensible  rapidity  to- 
ward an  unknown  goal.  Since  the  beginning  of  the  world,  the  earth 
has  not  twice  passed  the  same  spot,  and  the  place  which  we  occupy  at 
this  very  moment  is  rapidly  sinking  behind  into  our  track  never  to  re- 
turn. The  very  terrestrial  surface,  too,  undergoes  changes  every  centu- 
ry, every  year,  every  day,  and  the  conditions  of  life  change  throughout 
eternity  as  throughout  space.  After  having  thus  examined  the  move- 
ment of  the  earth  in  space,  we  must  join  to  it,  in  order  to  complete  its 
astronomical  aspect,  the  motion  of  the  moon  round  the  earth  in  twenty- 
nine  days  and  a  half.  The  moon  is  only  -£$  of  the  size,  and  -g^-  of  the 
weight  of  the  earth.  Its  action  upon  the  ocean  and  the  atmosphere  is, 
nevertheless,  comparable  with  that  of  the  sun,  and  is  even  more  impor- 
tant as  regards  the  production  of  tides :  it  is  as  useful  to  know  its  move- 
ment about  us  as  to  know  that  of  our  planet  about  its  primary.  The 
revolution  of  the  moon  around  the  earth  takes  place  really  in  twenty- 
seven  days  and  eight  hours,  but  during  these  twenty-seven  days  the 
earth  has  not  been  motionless,  but,  on  the  contrary,  has  advanced  a  cer- 
tain distance.  The  moon  employs  about  two  days  more  to  complete  its 
revolution  and  to  return  to  the  same  point  in  relation  to  the  sun,  which 
gives  twenty-nine  days  and  thirteen  hours  for  the  lunation  or  the  cycle 
of  phases.  The  revolution  in  twenty-seven  days  is  called  the  sidereal 
revolution,  because  in  that  time  the  moon  returns  upon  the  celestial 
sphere  to  the  same  position  in  relation  to  the  stars.  We  see  that  to  re- 
turn to  the  same  position  in  relation  to  the  sun,  and  to  accomplish  its 
synodical  revolution,  our  satellite  must  make  more  than  a  circle  upon 


22  THE  ATMOSPHERE. 

the  celestial  sphere,  and  pass  over  in  addition  the  distance  which  the 
earth  has  traveled  during  that  time.  If  we  suppose  the  earth  motion- 
less, the  movement  of  the  moon  round  it  may  be  nearly  represented  by 
a  circle.  In  reality,  it  is  a  sinuous  line,  resulting  from  the  combination 
of  the  two  movements. 

Three  stars  thus  command  our  attention  in  the  general  history  of  na- 
ture—the sun,  the  earth,  and  the  moon.  They  are  held  up,  isolated, 
in  space  in  a  manner  dependent  on  tfceir  respective  weights.  The  sun 
weighs  two  quadrillions  of  tons  (two  followed  by  twenty-four  zeros). 
The  sun  is  355,000  times  heavier  than  the  earth,'the  latter  eighty  times 
more  so  than  the  moon.  The  sun  holds  the  earth  at  arms-length,  so  to 
speak,  ninety-one  and  a  half  millions  of  miles  distant ;  the  earth  holds 
the  moon — also  by  the  influence  of  its  mass — at  a  distance  of  237,000 
miles. 

In  gravitating  around  our  luminary,  the  earth,  constantly  immersed 
in  its  rays,  brings  the  different  portions  of  its  surface  successively  into 
its  fertilizing  emanations.  Morning  succeeds  evening,  and  spring  au- 
tumn. Night,  like  winter,  is  but  the  transition  from  one  light  to  anoth- 
er. The  solar  heat  keeps  in  continual  work  the  mighty  factory  of  the 
terrestrial  atmosphere,  forming  the  currents,  the  winds,  the  tempests, 
and  the  breezes ;  preserving  the  water  liquid  and  the  air  gaseous,  rais- 
ing water  from  the  inexhaustible  wells  of  the  ocean,  producing  the 
mists,  the  clouds,  the  rains,  and  the  storms ;  organizing,  in  a  word, 
the  permanent  system  of  the  vital  circulation  of  the  globe. 

It  is  this  system  of  circulation,  with  the  varied  phenomena  of  the 
atmospheric  world,  which  we  are  about  to  study  in  this  work.  The 
subject  is  vast  and  grand,  for  upon  it  depends  all  terrestrial  life.  In 
studying  it  we  learn,  therefore,  the  very  organism  of  existence  upon 
the  planet  we  inhabit. 


THE  ATMOSPHERIC  ENVELOPE.  23 


CHAPTER  II. 

THE  ATMOSPHERIC   ENVELOPE. 

OUR  globe,  the  motions  of  which  we  have  been  explaining,  is  encir- 
cled by  a  gaseous  film  which  adheres  to  its  entire  spherical  surface. 
This  layer  of  fluid  extends  with  uniform  thickness  all  round  the  globe, 
covering  it  on  every  side.  We  have  already  compared  the  earth  in  the 
midst  of  space  to  a  cannon-ball  launched  into  the  air ;  by  imagining 
this  cannon-ball  surrounded  by  a  thin  ring  of  smoke  not  more  than  jfa 
of  an  inch  thick,  we  may  form  some  idea  of  the  position  of  the  atmos- 
phere around  the  terrestrial  globe.  It  is,  indeed,  from  this  position  that 
the  atmosphere  derives  its  name  ('ATJUOC,  vapor ;  and  >  S^aT/oa,  sphere), 
being,  as  it  were,  a  second  sphere  of  vapor  concentric  with  the  solid 
sphere  of  the  globe  itself.  As  a  rule,  sufficient  importance  is  not  at- 
tached to  the  functions  of  this  atmospheric  envelope.  It  is  from  it  that 
we  draw  our  beipg.  Plants,  animals,  and  men  imbibe  therefrom  the 
first  elements  of  their  existence.  The  earth's  organization  is  so  ordered 
that  the  atmosphere  is  sovereign  of  all  things,  and  that  'the  savant  can 
say  of  it  as  the  theologian  said  of  God:  "In  it  we  live  and  move,  and 
have  our  being." 

The  air  is  the  first  bond  of  society.  Were  the  atmosphere  to  vanish 
into  space,  an  eternal  silence  would  be  the  lot  of  the  terrestrial  surface. 
We  may  not  think  of  the  fact  with  our  forgetfulness  of  nature,  but  none 
the  less  the  air  is  the  great  medium  of  sound,  the  liquid  channel  in 
which  our  words  travel,  the  vehicle  of  language,  of  ideas,  and  of  social 
communication. 

It  is  also  the  first  element  of  our  bodily  tissues.  Breathing  affords 
three-quarters  of  our  nourishment;  the  other  quarter  we  obtain  in  the 
aliment,  solid  and  fluid,  in  which  oxygen,  hydrogen,  nitrogen,  and  car- 
bonic acid  are  the  chief  component  parts.  Further,  the  particles  which 
are  at  the  present  moment  incorporated  in  our  organism  will  make 
their  escape  either  in  perspiration  or  in  the  process  of  breathing;  and, 
after  having  sojourned  for  a  certain  time  in  the  atmosphere,  will  be  re- 
incorporated  in  some  other  organism,  either  of  plant,  animal,  or  man. 

With  the  unceasing  metamorphoses  in  beings  and  in  things,  there  is 


24  THE  ATMOSPHERE. 

at  the  same  time  going  on  a  continuous  exchange  between  the  products 
of  nature  and  the  moving  flood  of  the  atmosphere,  by  virtue  of  which 
the  gases  of  the  air  take  up  their  abode  in  the  animal,  the  plant,  or  the 
stone,  while  the  primitive  elements,  momentarily  incorporated  in  an 
organism,  or  in  the  terrestrial  strata,  effect  their  release  and  help  to  re- 
compose  the  aerial  fluid.  Each  atom  of  air,  therefore,  passes  from  life 
to  life,  as  it  escapes  from  death  after  death ;  being  in  turn  wind,  flood, 
earth,  animal,  or  flower,  it  is  successively  employed  in  the  composition 
of  a  thousand  different  beings.  The  inexhaustible  source  whence  ev- 
ery thing  that  lives  draws  breath,  the  air  is,  besides,  an  immense  reser- 
voir into  which  every  thing  that  dies  pours  its  last  breath ;  under  its 
action,  vegetables  and  animals  and  various  organisms  are  brought  into 
existence,  and  then  perish.  Life  and  death  are  alike  in  the  air  which 
we  breathe,  and  perpetually  succeed  the  one  to  the  other  by  the  ex- 
change of  gaseous  particles ;  thus  the  atom  of  oxygen  which  escapes 
from  the  ancient  oak  may  make  its  way  into  the  lungs  of  the  infant  in 
the  cradle,  and  the  last  sigh  of  the  dying  man  may  go  to  nourish  the 
brilliant  petal  of  a  flower.  The  breeze  which  caresses  the  blades  of 
grass  goes  on  its  way  until  it  becomes  a  tempest  that  uproots  the  forest- 
trees  and  strews  the  shore  with  shipwrecks;  and  so,  by  an  infinite  con- 
centration of  partial  death,  the  atmosphere  provides  an  unfailing  sup- 
ply of  aliment  for  the  universal  life  spread  over  the  surface  of  the 
earth. 

It  is  this  unceasing  activity  of  the  aerial  envelope  of  gas  which  forms, 
nourishes,  and  sustains  the  vegetable  carpet  that  extends  over  the  sur- 
face of  the  dry  land.  From  the  meanest  blade  of  grass  to  the  colossal 
Baobab,  this  rich  and  diversified  covering  draws  all  its  sustenance  from 
the  air. 

And  while  it  keeps  up  the  vital  circulation  of  the  earth  by  incessant 
exchanges  of  which  it  is  the  vehicle,  the  atmosphere  is  also  the  aerial 
laboratory  of  that  splendid  world  of  colors  which  brightens  the  surface 
of  our  planet.  It  is  owing  to  the  reflection  of  the  blue  rays  that  the 
sky  and  the  distant  heights  near  the  horizon  assume  their  lovely  azure 
tint,  which  varies  according  to  the  altitude  of  the  spot  and  the  abun- 
dance of  the  exhalations ;  and  to  it  also  we  owe  the  contrast  of  the 
clouds.  It  is  in  consequence  of  the  refraction  of  the  luminous  rays,  as 
they  pass  obliquely  across  the  aerial  strata,  that  the  sun  announces  its 
approach  every  morning  by  the  soft  and  pure  melody  of  the  glowing 
dawn,  and  makes  its  appearance  before  the  astronomical  hour  at  which 


THE  ATMOSPHERIC  ENVELOPE.  25 

it  should  rise ;  it  is  owing  to  a  similar  phenomenon  that,  toward  even- 
ing, it  apparently  slackens  the  speed  of  its  descent  beneath  the  horizon, 
and,  when  it  has  disappeared,  leaves  floating  upon  the  western  heights 
the  fantastic  fragments  of  its  blazoned  bed.  Without  the  gaseous  en- 
velope of  our  planet,  we  should  never  have  that  varied  play  of  light, 
those  changing  harmonies  of  color,  those  gradual  transformations  of 
delicate  shades  which  lighten  up  the  world,  from  the  gleaming  bright- 
ness of  the  summer  sun  down  to  the  shadows  which  cover,  as  with  a 
veil,  the  forest  depths. 

The  study  of  the  atmosphere  embraces  also  the  general  conditions  of 
terrestrial  existence.  The  notion  of  life  is  so  bound  up  in  all  our  con- 
ceptions with  that  of  the  forces  which  we  see  ever  at  work  in  nature, 
that  the  myths  of  the  early  inhabitants  of  the  world  always  attributed 
to  these  forces  the  generation  of  plants  and  animals,  and  imagined  the 
epoch  anterior  to  life  as  that  of  primitive  chaos  and  struggle  of  the 
elements.  "  If  we  do  not  consider,"  says  Humboldt,  "  the  study  of  phys- 
ical phenomena  so  much  as  bearing  on  our  material  wants  as  in  their 
general  influence  upon  the  intellectual  progress  of  humanity,  it  will  be 
found  that  the  highest  and  most  important  result  of  our  investigation 
will  be  the  knowledge  of  the  intercommunication  of  the  forces  of  na- 
ture, and  the  certainty  of  their  mutual  dependence  upon  each  other. 
It  is  the  perception  of  these  relations  which  enlarges  the  views  and  en- 
nobles our  enjoyment  of  them.  This  enlargement  of  the  view  is  the 
result  of  observation,  of  meditation,  and  of  the  spirit  of  the  age  in  which 
all  the  directions  of  thought  concentrate  themselves.  History  teaches 
him  who  can  travel  back  through  the  strata  of  preceding  centuries  to 
the  farthest  roots  of  knowledge  how,  for  thousands  of  years,  the  human 
race  has  labored  to  grasp,  through  ever-recurring  changes,  the  fixity  of 
the  laws  of  nature,  and  to  gradually  conquer  a  large  portion  of  the 
physical  world  by  the  force  of  intelligence." 

The  most  important  result  of  a  rational  examination  of  nature  is,  that 
it  leads  one  to  comprehend  unity  and  harmony  in  this  immense  assem- 
bly of  things  and  forces,  to  embrace  with  equal  ardor  what  is  due  to  the 
discoveries  of  past  ages  and  to  those  of  our  own  time,  and  to  analyze 
the  details  of  phenomena  without  succumbing  beneath  their  weight. 
It  is  thus  that  it  has  been  given  to  man  to  show  himself  worthy  of 
his  high  destiny,  by  penetrating  into  the  meaning  of  nature,  unveiling 
its  secrets,  and  mastering  by  thought  the  materials  collected  by  obser- 
vation. 


2g  .  THE  ATMOSPHERE. 

We  may  now  contemplate  our  planet  traveling  in  space,  and  keeping 
about  it  the  aerial  envelope  which  adheres  to  its  surface.  Our  imagina- 
tion can  easily  comprehend  the  general  shape  of  this  gaseous  sphere 
which  encircles  the  solid  globe,  and  which  is  comparatively  thin  and  of 
slight  bulk. 

The  exterior  surface  of  the  atmosphere  is  therefore  curved  like  that 
of  the  sea,  for,  like  water,  the  external  layer  of  air  tends  to  a  level,  all 
points  of  which  are  at  equal  distances  from  the  centre.  To  the  eyes  of 
novices,  it  seems  difficult  to  reconcile  the  idea  of  the  spherical  surface  of 
the  ocean  with  what  is  commonly  termed  a  level;  the  idea  that  the  air 
has  a  horizontal  level  like  water,  and  that,  like  an  aerial  ocean,  this 
level  is  always  tending  to  an  equilibrium,  seems  at  first  sight  somewhat 
obscure.  Nevertheless,  not  only  does  the  air  possess  to  an  unlimited 
degree  all  the  properties  of  elasticity  and  mobility  of  a  fluid  seeking 
equilibrium,  but,  different  in  this  respect  from  water  and  other  liquids, 
it  is  extremely  capable  of  compression  and,  consequently,  susceptible  of 
extreme  expansion.  These  are  facts  which  must  always  be  kept  in 
mind,  for  they  will  assist  in  the  understanding  of  a  great  number  of  at- 
mospheric conditions  explained  in  future  chapters  of  this  work. 

What,  then,  is  the  thickness  of  this  gaseous  stratum  which  envelops 
our  globe?  This  is  the  point  which  we  shall  examine  in  the  next 
chapter. 

To  ascertain  the  height  to  which  the  atmosphere  extends,  it  would  be 
necessary  to  calculate  the  density  of  the  air  at  different  elevations  in  the 
average  state,  leaving  out  of  consideration  accidental  disturbances.  This 
can  be  done  when  we  know  the  temperature  of  the  air,  its  pressure,  and 
the  tension  of  the  vapor  of  water  which  it  contains.  It  would,  further, 
be  necessary,  in  order  to  obtain  an  exact  determination,  to  take  account, 
first,  of  the  gradual  diminution  in  weight  as  the  distance  from  the  cen- 
tre of  the  earth  is  increased ;  secondly,  of  the  variation  in  the  centrifu- 
gal force  according  to  the  latitude.  These  variations  are,  however,  slight, 
and  scarcely  affect  the  calculation,  in  consequence  of  the  coat  of  air  being 
of  such  insignificant  thickness  as  compared  to  the  radius  of  the  globe. 

The  height  of  the  atmosphere  has  its  limits,  which,  as  we  shall  see,  are 
somewhat  confined.  If  the  air  had  no  elasticity,  its  limit  would  be  at  a 
distance  where  the  centrifugal  force  was  in  equilibrium  with  the  weight; 
but  as  this  condition  does  not  exist,  its  elasticity  must  necessarily  be  coun- 
terbalanced by  a  force  of  some  kind,  and  this  force  is  the  weight  of  the 
strata  of  air  which  are  above  the  particular  one  we  are  considering.  But 


THE  ATMOSPHERIC  ENVELOPE.  27 

the  higher  we  ascend  the  more  rarefied  does  the  air  become,  and  when 
the  last  strata  are  reached  there  is  nothing  to  keep  them  down.  Nev- 
ertheless, the  atmosphere  being  limited,  as  we  shall  presently  see,  these 
strata  can  not  be  lost  in  space ;  and  it  is  probable  that  in  consequence  of 
their  rarefaction  and  the  great  decline  in  their  temperature,  their  phys- 
ical condition  is  so  modified  that  the  elastic  force  becomes  nil.  Laplace 
has  pointed  out  this  indispensable  condition ;  Poisson  has  specified  it,  by 
showing  that  the  equilibrium  would  still  be  possible  with  a  very  consid- 
erable limiting  density,  provided  that  the  fluid  was  not  capable  of  ex- 
pansion ;  and  Biot,  who  has  summed  up  these  conditions,  clearly  indi- 
cates the  state  of  these  external  inexpansible  strata  in  his  remark  that 
they  must  be  like  "  a  liquid  which  does  not  evaporate."  We  will  now 
examine  the  mechanical  and  physical  conditions  of  this  aerial  envelope, 
estimate  its  exterior  shape,  and  measure  its  height. 


23  THE  ATMOSPHERE. 


CHAPTER  III. 

THE  HEIGHT  OF  THE  ATMOSPHERE. 

As  the  earth  travels  in  space  with  enormous  swiftness,  carrying  along 
with  it,  adhering  to  its  surface,  the  gaseous  body  that  encircles  it,  it 
naturally  follows  that  this  latter  does  not  extend  indefinitely  into  space, 
but  ceases  to  exist  at  a  certain  distance  from  the  surface.  How  far 
can  it  extend  ?  Carried  along  by  the  rotation  of  the  globe  in  its  daily 
movement,  we  may  conclude  that  at  a  certain -height  above  the  ground 
the  movement  of  the  atmosphere  is  so  rapid  that  the  centrifugal  force 
which  it  acquires  would  hurl  into  space  the  outside  particles  of  air, 
which  would  then  cease  to  adhere  to  the  surface  and,  for  the  same  rea- 
son, to  form  part  of  the  atmosphere. 

Certain  inventors  of  methods  of  aerial  navigation  have  vaguely  im- 
agined that  the  atmosphere  does  not  entirely  turn  round  with  the  earth, 
so  that,  by  rising  to  a  certain  height,  we  could  see  the  globe  moving 
around  beneath  our  feet,  and  should  only  have  to  wait  until  the  me- 
ridian, where  we  wished  to  alight,  passed  under  the  balloon,  to  find 
ourselves  transported  there  by  the  rotation  of  the  globe.  Such  an  idea 
is,  of  course,  absurd,  as  the  atmosphere  and  all  that  it  contains  partake 
equally  with  the  earth  in  the  rotation  of  the  latter. 

The  centrifugal  force  increases  as  the  square  of  the  velocity,  and  at 
the  equator  its  amount  is  -^  part  of  that  of  gravity,  so  that  a  body  at 
the  equator  weighs  less  than  the  same  body  at  either  of  the  poles  by 
-j-g-g-  of  its  weight.  If,  therefore,-  the  earth  rotated  on  its  axis  seventeen 
times  as  fast  as  it  does,  since  seventeen  times  seventeen  is  equal  to  two 
hundred  and  eighty-nine,  a  body  at  the  equator  would  not  have  any 
weight.  A  stone,  for  instance,  detached  from  the  ground  by  the  action 
of  the  hand,  would  not  fall  down  again ;  we  should  become  so  feath- 
er-like, that,  in  dancing  upon  the  surface,  we  should  resemble  aerial 
nymphs  displaced  by  the  wind.  As  the  circumferences  of  circles  vary 
as  their  radii,  at  seventeen  times  the  distance  from  the  surface  to  the 
centre  of  the  earth— that  is  to  say,  at  a  height  of  about  sixteen  times 
the  radius  of  the  earth,  or  about  63,000  miles— if  the  other  quantities 
involved  remained  unchanged,  the  atmosphere  would  cease  to  rotate 


THE  HEIGHT  OF  THE  ATMOSPHERE. 


29 


with  the  earth;  but,  in  point  of  fact,  the  weight  does  not  remain  un- 
changed, but  diminishes  as  the  distance  from  the  centre  of  attraction  is 
increased. 

By  combining  this  diminution  with  the  increase  of  centrifugal  force, 
we  find  that  at  a  distance  of  about  6"61  times  the  radius  of  the  earth 
from  its  centre,  which  corresponds  to  a  height  above  its  surface  of  about 
21,000  miles,  the  centrifugal  force  is  equal  to  the  weight,  and  conse- 
quently the  aerial  particles  which  might  happen  to  be  in  these  regions 
must  of  necessity  escape.  This  is  the  distance  at  which  a  satellite 
would  gravitate  in  exactly  twenty-three  hours  fifty-six  minutes,  the 
time  occupied  by  our  planet  in  its  rotation.  It  is,  theoretically,  the  max- 
imum limit  of  the  atmosphere,  which,  however,  as  a  matter  of  fact,  is 
far  from  extending  to  so  great  a  height,  as  we  shall  see;  but,  mathe- 
matically, it  might  do  so,  and  it  is  only  at  this  enormous  distance  that 
the  centrifugal  force  would  be  sufficiently  great  to  prevent  the  atmos- 
phere from  existing  as  such. 

Such  is  the  extreme  and  maximum  limit  of  the  atmosphere;  but  it  is 
at  a  far  lower  elevation  that  the  air  we  breathe  really  ceases.  Thus,  at 
the  height  of  10,000  feet — the  height  of  Mount  ^Etna — there  is  beneath 
the  mountaineer  nearly  a  third  of  the  aerial  mass ;  at  18,000  feet,  which 
is  less  than  that  of  the  peaks  of  many  mountains,  the  column  of  air 
which  presses  upon  the  soil  has  already  lost  half  its  weight,  and  conse- , 
quently  at  this  point  the  whole  gaseous  mass,  which  reaches  far  up  into 
the  sky,  does  not  weigh  more  than  the  strata  which  are  compressed  into 
the  region  below. 

In  consequence  of  the  forces  that  act  upon  it,  the  shape  of  the  atmos- 
phere is  not  absolutely  spherical, 
but  swollen  out  at  the  equator, 
where  it  is  much  higher  than  at 
the  poles.  The  maximum  limit 
of  this  figure,  in  the  case  where 
the  flattening  is  greatest,  has  been  E 
given  by  Laplace.  The  diameter 
of  the  atmosphere  at  the  equator 
is  a  third  greater  than  at  the 
poles.*  It  is  the  mathematical 


limit,  beyond  which  the  terrestrial 


Fig.  1 — Mathematical  limit  of  the  shape  of  the  at- 
mosphere. 

*  [This  is  inaccurate.     Laplace  proved  that  the  ratio  of  the  least  (the  polar)  diameter  to  the 
greatest  (the  equatorial)  diameter  could  not  be  less  than  §  (not  f,  as  in  the  text).     Fig.  1  is 


THE  ATMOSPHERE. 


30 

atmosphere  can  not  pass.  But  it  has  not  this  exaggerated  shape, 
though  in  reality  it  is  perceptibly  denser  at  the  equator  than  at  the 
poles.  It  may  be  remarked  that  it  is  probable  that  a  detached  train 
of  the  lighter  gases  remains  constantly  in  the  rear  of  the  globe  during 
its  rapid-  revolution  around  the  sun.  It  need  scarcely  be  added  that 
the  shape  of  the  atmosphere  undergoes  further  change,  owing  to  the 
atmospheric  tides,  which  are  due  to  the  varying  attraction  of  the  sun 
and  the  moon. 

The  decreasing  weight  of  the  atmospheric  strata  affords  us  the  first 
means  of  calculating  a  minimum  limit  of  the  height  of  the  atmosphere. 
Mechanics  have  given  us  the  maximum  limit,  and  it  is  in  this  instance 
to  physics  that  we  shall  have  recourse. 

Consider  a  vertical  column  of  air,  then  the  pressure  at  any  point 
must  be  equal  to  the  weight  of  air  above ;  or,  in  other  words,  any  por- 
tion of  the  column  measured  from  the  ground  supports  all  the  rest  of 
the  column  above;  the  lower  strata  of  the  atmosphere  are,  therefore, 
more  pressed  down  (and  consequently  denser),  because  they  have  a 
greater  weight  resting  on  them.  The  barometer,  which  measures  this 
pressure  of  the  air,  is  higher  at  the  foot  than  at  the  summit  of  a  mount- 
ain ;  and  the  relation  which  exists  between  the  pressure  and  the  height 
is  so  close,  that  the  difference  in  level  between  two  points  may  be  de- 
-duced  from  the  difference  in  the  heights  of  the  barometrical  columns 
simultaneously  placed  at  these  two  stations.  The  smaller  the  pressure 
the  more  dilated  is  the  air ;  so  that,  at  first  sight,  it  would  seem  as  if  the 
atmosphere  must  extend  to  an  immense  distance. 

A  celebrated  natural  philosopher,  Mariotte,  first  determined  the  law 
of  the  compression  of  gases ;  and  the  result  of  his  researches  shows  that 
the  quantity  of  air  contained  in  the  same  volume — or,  in  other  words, 
the  density  of  the  air — is  proportionate  to  the  pressure  to  which  it  is 
subjected.  Until  within  the  last  few  years  this  law  was  considered  en- 
tirely accurate;  but  recently  it  has  appeared  most  difficult  to  conceive 
why  the  terrestrial  atmosphere  does  not  extend  very  far  into  space ; 
while  other  considerations  indicate  that  it  is  necessarily  limited,  and 

therefore  incorrectly  drawn,  as  the  protuberance  should  be  considerably  greater.  It  may  be 
mentioned  that  one  consequence  deduced  by  Laplace  from  his  result  is,  that  the  Zodiacal  light 
can  not  be  produced  by  reflection  on  the  atmosphere  of  the  sun,  as  the  former  always  appears 
in  the  form  of  a  thin  lens,  the  ratio  of  the  polar  to  the  equatorial  diameter  being'much  less 
than  §.  Laplace's  investigation  is  given  in  vol.  ii.,  pp.  194-197  of  the  Mecanigue  Celeste 
(National  Edition).— ED.] 


THE  HEIGHT  OF  THE  ATMOSPHERE.  31 

ceases  at  a  short  distance  above  the  ground.  This  apparent  contra- 
diction was  the  result  of  a  too  extensive  generalization  of  Mariotte's 
law,  which  is  simply  relative  instead  of  rigorously  definite;  and  Eeg- 
nault  has  studied  the  differences  which  exist  between  the  theoretical 
law  and  the  facts  of  the  case. 

Subsequently  to  these  investigations,  M.  Liais  has  ascertained,  by  in- 
troducing very  small  portions  of  air  into  a  large  barometrical  instru- 
ment made  for  the  purpose,  that  the  differences  between  the  results  of 
observation  and  the  theory  usually  adopted  are  still  greater.  By  di- 
minishing sufficiently  the  quantity  of  air,  it  has  been  possible  to  find  a 
limit  at  which  the  particles,  far  from  separating  from  each  other,  as 
would  happen  were  the  gases  capable  of  indefinite  dilatation,  seem,  on 
the  contrary,  to  have  a  mutual  tendency  to  adhesion  similar  to  that  of 
the  molecules  in  a  viscous  liquid.  The  elasticity  of  the  air,  producing 
expansion,  ceases,  therefore,  at  a  certain  degree  of  dilatation,  from  which 
point  this  gas  assumes  the  character  of  a  liquid,  but  a  liquid  out  of  all 
comparison  lighter  than  those  with  which  we  are  acquainted. 

By  means  of  this  decrease  in  the  density  of  the  air  in  proportion  to 
its  height,  Biot  has,  by  an  examination  of  the  physical  conditions  of 
equilibrium  and  a  complete  discussion  of  the  observations  obtained  at 
different  degrees  of  altitude  by  Gay-Lussac,  Humboldt,  and  Boussin- 
gault,  demonstrated  that  the  minimum  height  of  the  atmosphere  is 
160,000  feet,  or  about  thirty  miles.  At  that  height  the  air  must  be  as 
rarefied  as  beneath  the  exhausted  receiver  of  an  air-pump;  that  is  to 
say,  as  rarefied  as  the  air  in  the  nearest  approach  to  a  vacuum  that  we 
can  make. 

Thus  the  minimum  height  of  the  atmosphere  is  thirty  miles,  and  the 
maximum  21,000.  Hence  we  have  two  defined  limits,  but  with  a  great 
distance  between  them.  There  are,  however,  other  methods  by  which 
we  can  get  nearer  to  the  truth.  Efforts  have  been  made  to  measure  the 
height  of  the  atmosphere  optically,  by  studying  the  length  of  the  twi- 
light and  the  length  of  time  during  which  the  solar  rays  continue  to 
reach  the  aerial  regions  when  the  luminary  himself  has  sunk  below  the 
horizon. 

If  the  atmosphere  were  unlimited,  the  phenomenon  of  night  would 
be  entirely  unknown  to  us;  the  light  of  the  sun,  reaching  the  strata  of 
air  which  are  sufficiently  distant  from  the  earth,  would  be  continuously 
sent  on  to  us  by  reflection  from  these  strata.  On  the  other  hand,  the 
absence  of  any  aerial  envelope  would  cause  the  night  to  begin  exactly 


32 


THE  ATMOSPHERE. 


at  sunset  and  the  light  of  day  to  burst  upon  us  immediately  the  sun 
rose  As  it  is,  every  one  knows  that  the  twilight  of  evening  and  the 
morning  dawn  prolong  the  time  during  which  we  enjoy  the  solar  light. 
It  will  be  readily  imagined  that  the  observation  of  these  phenomena  at 
once  suggested  the  idea  of  seeking  to  resolve,  by  their  agency,  the 
height  to  which  the  atmosphere  extended. 

Suppose  the  earth  to  be  represented  by  the  circle,  radius  o  A,  and 
that  its  atmosphere  is  limited  by  the  circumference  F  G  H  I  c.  It  is 
evident  that,  when  the  sun  has  sunk  beneath  the  horizon  F  A  c  B  of  the 

place  A,  it  will  only  give 
light  to  a  portion  of  the 
atmosphere.  Thus,  when 
the  sun  arrives  at  j,  if 
we  imagine  a  tangent 
cone  to  the  earth,  hav- 
ing the  sun  for  its  sum- 
mit, all  those  parts  of 
the  atmosphere  situated 
below  J  G  will  be  de- 
prived of  light,  and  the 
part  c  I  H  G  will  alone 

Fi"  2  -Measure  of  the  height  of  the  atmosphere,  according  to    be     illuminated.        Later 

on,  when  the  sun  reaches 

j',  the  portion  bounded  by  c  I  H  will  alone  be  subject  to  its  light; 
later  still,  only  from  c  to  I ;  and  finally,  when  the  sun  gets  to  j'",  upon 
the  tangent  line  from  c,  the  intersection  of  the  plane  of  the  horizon 
FACE  and  the  limiting  sphere  of  the  atmosphere,  the  twilight  ceases. 
From  the  moment,  therefore,  that  the  sun  sets,  we  ought  to  see  a  sort 
of  arc  appear  on  the  opposite  side  of  the  horizon,  rising  gradually  until 
it  reaches  the  zenith,  and  then  slowly  descend  until  it  finally  disap- 
pears. Such  is  the  theory  that  the  earliest  astronomers  conceived  as 
to  the  phenomenon  of  twilight.  In  the  optics  of  Alhasen  (in  the  tenth 
century)  we  find  that  the  angle  of  the  sun's  declivity  for  the  close  of  the 
twilight  or  the  break  of  dawn  was  taken  as  18°,  and  this  estimate  is 
still  adopted  by  modern  astronomers  as  the  average  amount. 

In  our  climate  it  is  difficult  to  distinguish  with  accuracy  the  limit  of 
separation  between  that  part  of  the  atmosphere  which  is  lighted  by  the 
sun  and  that  which  does  not  receive  its  rays  directly.  But  Lacaille,  in 
his  voyage  to  the  Cape  of  Good  Hope,  recognized  all  the  phases  which 


THE  HEIGHT  OF  THE  ATMOSPHERE.  33 

have  been  enumerated  theoretically.  He  says:  "Upon  the  16th  and 
17th  of  April,  1751,  while  at  sea  and  in  calm  weather,  the  sky  being  ex- 
tremely clear  and  serene,  at  the  point  where  I  could  distinguish  Venus 
at  the  horizon  as  a  star  of  the  second  magnitude,  I  saw  the  twilight  ter- 
minated in  the  arc  of  a  circle  as  regularly  as  possible.  Having  regula- 
ted my  watch  by  the  exact  hour,  according  to  sunset,  I  saw  this  arc  lost 
in  the  horizon,  and  I  calculated,  by  the  hour  at  which  I  made  this  ob- 
servation, that  the  sun  had  descended  below  the  horizon,  on  the  16th 
of  April,  16°  38',  and  on  the  17th,  17°  13V 

Other  observations  have  since  been  made,  as  we  shall  see  further  on. 

It  is  easy  to  understand  that,  once  having  ascertained  the  apparent 
daily  circle  described  by  the  sun  upon  a  certain  date,  and  the  position 
of  the  observer  upon  the  earth,  we  can  calculate,  by  the  time  that  has 
elapsed  between  the  hour  of  sunset  and  the  moment  of  the  crepuscular 
arc's  disappearance,  the  angle  traversed  by  the  sun  below  the  hori- 
zon. It  will  also  be  understood  that,  according  to  the  time  and  place, 
there  will  be  found  a  difference  both  in  regard  to  twilight  and  dawn, 
since  the  variations  in  the  relative  position  of  the  sun  and  the  state 
of  the  air  must  necessarily  influence  the  direction  and  quantity  of  the 
light  which,  after  countless  reflections  and  refractions,  reaches  the  ob- 
server. 

We  will  study,  in  the  second  book,  the  optical  effects  of  twilight;  at 
present  we  are  only  concerned  with  -the  relation  existing  between  its 
duration  and  the  height  of  the  atmosphere. 

Now,  the  time  during  which  the  sun,  after  sinking  below  the  horizon 
of  a  particular  spot,  continues  to  give  light  directly  to  part  of  the  at- 
mosphere visible  from  this  place,  depends  upon  the  thickness  of  the 
aerial  strata  which  envelop  the  earth.  Let  us  suppose,  for  instance, 
that  we  pass  a  plane  (Fig.  2)  through  the  place  A,  the  centre,  o,  of  the 
earth  and  the  centre  of  the  sun ;  this  plane  will  cut  the  earth  in  the  cir- 
cle o  A.  Let  F  A  B  be  the  intersection  of  the  horizon  of  the  spot  A  with 
this  same  plane ;  from  c  draw  the  tangent  C  D  to  the  earth ;  all  that 
part  of  the  atmosphere  visible  at  A  will  cease  to  be  illuminated  by  the 
sun  when,  in  its  apparent  diurnal  movement,  it  has  sunk  below  CD  j'". 
Now  we  have  seen  that,  from  the  duration  of  twilight,  it  was  concluded 
that  it  came  to  an  end  when  the  angle  BC  j'"  of  descent  below  the  ho- 
rizon was  18°.  As  the  angle  o  A  c  is  a  right  angle,  and  as  o  A  is  the 
radius  of  the  earth,  we  know  one  side  and  the  angles  of  the  triangle 
o  AC,  and  consequently  are  enabled  to  calculate  the  other  parts,  oc 

3 


34  THE  ATMOSPHERE. 

may  therefore  be  regarded  as  known,  and  thence  it  results  that  we  have 
the  height,  E  c,  of  the  atmosphere,  for  E  c  =  o  c— o  E. 

Such  is  the  method  devised  by  Kepler  for  deducing  the  height  of  the 
atmosphere  from  the  phenomena  of  twilight.  The  results  which  it  has 
furnished  agree  with  the  preceding,  and  give  our  atmosphere  a  height 
of  from  thirty  to  thirty-seven  miles.*  The  average  radius  of  the  earth 
being  3908  miles,  it  will  be  seen  that  this  height  is  but  a  little  more 
than  the  130th  part  of  this  radius;  that  is  to  say,  that  if  the  earth  were 
represented  by  a  sphere  about  twenty-two  feet  in 
diameter,  the  atmosphere  would  be  like  a  coat  of 
vapor  adhering  to  the  surface,  with  a  thickness  of 
about  one  inch. 

Figure  3  represents  exactly  this  relation.  It 
shows — firstly,  the  incandescent  interior  of  the 
globe,  which  is  a;  secondly,  the  solid  crust,  b,  on 
which  we  live  (it  is  but  twelve  leagues,  or  thirty 
miles,  thick,  as,  in  consequence  of  the  increased  tem- 
perature of  one  degree  (Fahrenheit)  for  fifty  feet, 
minerals  fuse  at  this  depth)  ;f  thirdly,  the  thickness 
of  the  aerial  layer  which  we  breathe,  and  which  is 
represented  by  c;  and,  fourthly,  the  probable  height 
of  a  very  light  atmosphere,  d,  over  and  above  ours, 
of  which  we  are  about  to  treat. 

It  may  be  further  mentioned,  in  reference  to  the 

ig.  S— Section     showing  J 

the  relative  thickness  of  measurement  of  the  height  of  the  atmosphere  by 

the  earth's  crust,  of  our  .  .  , 

atmosphere,  and  of  a  the  duration  of  twilight,  that  certain  observers  have 
higher  atmosphere.         obtained,  as  the  result  of  similar  researches,  an  ele- 
vation much  greater  than  that  given  above,  affording  a  clear  proof  that 
ihe  twelve  leagues  actually  represents  the  minimum  only.    M.  Liais  has 
made  a  direct  calculation  of  this  height  by  observing  the  duration  of 

*  [It  is  to  be  noted  that  different  methods  give  different  heights  for  the  atmosphere,  but 
ihere  is  no  discrepancy,  as  different  things  are  meant.  Thus,  if  experiments  on  twilight  give 
forty  miles  as  the  height,  this  implies  that  the  air  above  this  elevation  reflects  no  appreciable 
;:mount  of  light ;  while,  if  we  define  the  height  to  be  to  the  point  where  the  friction  will  not 
•et  light  to  a  meteor,  we  have  about  seventy  miles;  but,  of  course,  there  is  no  reason  why  there 
:  hould  not  be  some  air  at  much  greater  heights. — ED.] 

t  [This  is  the  observed  rate  of  decrease  at  the  surface  of  the  earth,  but  it  is  not  true  that 
:he  thickness  of  the  crust  must  be  as  stated  in  the  text.  It  follows,  from  several  considera- 
lions  of  other  kinds,  that  the  thickness  of  the  crust  is  in  all  probability  not  less  than  600 
miles.— ED.] 


THE  HEIGHT  OF  THE  ATMOSPHERE.  85 

twilight  and  of  the  crepuscular  curve,  which  colors  the  sky  with  that 
lovely  rose  tint  which  is  so  remarkable,  especially  in  southern  countries. 
These  observations  have  been  made  both  on  the  Atlantic,  during  a 
voyage  from  France  to  Eio  Janeiro,  and  in  the  bay  upon  the  shores 
of  which  the  last-named  city  stands.  They  give,  as  a  minimum,  180 
miles,  and,  as  a  probable  height,  204  miles. 

By  observing,  from  the  summit  of  the  Faulhorn,  the  course  of  the 
crepuscular  arcs,  Bravais  obtained  a  height  of  seventy-one  and  a  half 
miles.  The  height,  however,  varies  according  to  the  temperature  of  the 
seasons,  and  remains  always  greatest  at  the  equator.  Another  method, 
different  from  the  preceding,  consists  in  measuring  the  thickness  of  the 
penumbra  which  surrounds  the  earth's  shadow  on  the  moon  during  lu- 
nar eclipses,  as  well  as  the  phenomena  of  refraction  produced.  This 
measurement  gives  from  fifty  to  sixty  miles  as  the  thickness  of  the 
terrestrial  atmosphere,  the  influence  of  which  is  felt  under  this  special 
aspect. 

The  observations  which  accord  the  atmosphere  a  height  far  greater 
than  the  theoretical  thirty-eight  miles  have  been  for  many  years  the 
object  of  special  discussion.  Quetelet,  director  of  the  Brussels  Observa- 
tory, has,  after  much  research  on  this  head,  arrived  at  the  conclusion 
that  it  does  indeed  extend  much  higher  than  had  been  supposed,  but 
that  the  upper  strata  are  not  quite  of  the  same  nature  as  those  nearer 
the  earth. 

This  addition  is  supposed  to  be  due  to  an  ethereal  atmosphere,  very 
rarefied  and  differing  from  the  lower  atmosphere  in  which  we  live.  It 
is  the  region  where  are  mostly  seen  the  shooting  stars,  which  afterward 
disappear  when  they  reach  the  terrestrial  atmosphere. 

The  upper  atmosphere*  is  still,  the  lower  in  continual  motion.  The 
special  movements  caused  by  the  action  of  the  winds  and  tempests  are 
limited  in  their  height  by  the  effect  of  the  seasons.  Thus,  as  regards 
our  climate,  the  agitated  portion,  in  the  vicinity  of  the  earth,  would  not 
be  more  than  from  seven  to  ten  miles  high  during  the  winter,  while  its 
height  must  be  almost  double  in  summer.  All  that  part  of  the  atmos- 
phere which  is  above  the  latter  would  only  experience  a  very  slight 
and  scarcely  sensible  movement,  arising  from  the  movable  basis  upon 
which  it  reposes. 

The  continual  disturbances  going  on  in  the  lower  regions  cause  the 
air  in  the  inferior  atmosphere  to  be  very  much  alike  in  its  chemical 

*  [The  existence  of  such  an  atmosphere  seems  to  me  very  uncertain. — ED.] 


gg  THE  ATMOSPHERE. 

components.  No  difference  has  been  discovered  at  the  various  eleva- 
tions which  it  is  possible  to  attain  for  the  purpose  of  collecting  air  and 
submitting  it  to  analysis. 

In  the  upper  atmosphere  the  phenomena,  of  which  we  are  scarcely 
able  to  form  an  idea  by  judging  them  from  the  surface  of  our  globe, 
take  place.  There,  also,  appear  the  shooting  stars;  descending  from 
a  still  greater  height,  the  aurora  borealis,  and  those  mighty  luminous 
phenomena  which  we  often  witness  without  having  the  power  to  sub- 
mit them  directly  to  the  test  of  experiment.  All  these  facts  do  not  es- 
cape us  altogether,  especially  as  regards  the  aurora  borealis  and  the 
magnetic  phenomena.  If  we  can  not  determine  the  cause,  we  can  at 
least  feel  the  effect  with  sufficient  force  to  be  in  a  position  to  appre- 
ciate them. 

Sir  John  Herschel,  De  la  Eive,  and  Hansteen  seem  to  share  upon  this 
point  the  opinion  of  Quetelet.  We  can  quite  admit  that,  above  our  at- 
mosphere ot  oxygen,  nitrogen,  and  vapor  of  water,  there  exists  an  at- 
mosphere excessively  light,  which  may  extend  two  hundred  miles  in 
height,  and  which  is  naturally  composed  of  the  very  lightest  gases. 

The  terrestrial  globe  being  about  8000  miles  in  diameter,  this  total 
thickness  represents  the  fortieth  of  the  globe's  diameter.  The  simulta- 
neous existence  of  these  two  atmospheres  is,  therefore,  the  general  con- 
clusion at  which  we  will,  momentarily  at  least,  stop. 

As  to  the  basis  of  the  atmosphere,  we  may  now  inquire  if  it  ceases 
at  the  surface  of  the  ground,  and  does  not  descend  into  the  interior  of 
the  globe  itself. 

Pressing  upon  all  bodies  upon  the  surface  of  the  earth,  it  tends  to 
penetrate  in  all  directions  between  the  molecules  of  liquids  as  into  the 
interstices  of  the  rocks.  It  is  to  be  found  in  water  as  in  all  vegetables 
and  all  organic  structures ;  the  earth  and  the  porous  stones  are  impreg- 
nated with  it,  and  that  in  proportion  to  the  force  with  which  it  presses. 
It  will  be  seen,  therefore,  that  the  air  is  not  limited  to  the  part  which 
is,  so  to  speak,  a  gaseous  envelope,  and  that  a  sensible  fraction  of  its 
constituent  elements  penetrates  the  waters  of  the  ocean  and  the  inter- 
stices of  the  ground.  Certain  savants  have  imagined  that  the  air  of 
which  the  atmosphere  is  composed  is  but  the  continuation  of  an  inte- 
rior atmosphere ;  but  the  rise  in  the  temperature,  due  to  the  central 
heat,  would  prevent  the  condensation  of  gases,  and  must  limit  the  pres- 
ence of  air  in  the  under  strata. 

A  rough  estimate  of  the  quantity  of  air  which  is  thus  introduced  into 


THE  HEIGHT  OF  THE  ATMOSPHERE.  37 

the  waters  of  the  ocean  may  be  formed  by  measuring  the  absorption  of 
gases  by  various  liquids.  Under  ordinary  pressure,  sea-water  absorbs 
from  two  to  three  per  cent,  of  its  volume,  only  the  proportion  of  oxygen 
is  much  greater  than  in  the  ordinary  air.  The  result  of  the  calculation 
is,  that  the  quantity  of  air  absorbed  by  the  ocean  is  not  above  a  three- 
hundredth  part  of  the  atmosphere. 

We  thus  have  a  tolerably  complete  determination  both  as  to  the 
height  and  shape  of  this  terrestrial  atmosphere. 


OQ  THE  ATMOSPHERE. 

oo 


CHAPTER  IV. 

WEIGHT  OF  THfc  TERRESTRIAL  ATMOSPHERE-THE  BAROMETER  AND 
ATMOSPHERIC   PRESSURE. 

WHILE  treating  of  the  height  of  the  atmosphere,  we  have  already  seen 
that  the  air  is  denser  in  the  lower  regions  of  the  aerial  ocean-that  is  to 
say,  near  the  surface  of  the  earth— than  in  the  higher  regions.  The  air, 
light  and  unsubstantial  as  it  may  appear  to  us  to  be,  has  consequently  a 
positive  weight.  Each  square  foot  of  the  earth's  surface  sustains  a  con- 
siderable pressure,  the  amount  of  which  we  shall  presently  attempt  to 
estimate,  corresponding  to  the  height  and  density  of  the  column  of  air 

above  it. 

Our  ancestors  were  not  able  to  measure  the  atmospheric  pressure;  but 
we  must  not  conclude  from  this  that  they  were  ignorant  of  the  effects 
which  it  exercised,  especially  when  the  wind  was  violent.  Yet  this 
force,  which  every  one  felt  without  being  able  to  measure,  was  not  ren- 
dered determinate  until  the  middle  of  the  seventeenth  century. 

In  1640,  the  Grand  Duke  of  Tuscany  having  ordered  the  construc- 
tion of  fountains  upon  the  terrace  of  the  palace,  if  was  found  impossible 
to  make  the  water  rise  more  than  thirty-two  feet.  The  duke  wrote  to 
Galileo  in  reference  to  this  strange  refusal  of  the  water  to  obey  the 
pumps.  Torricelli,  the  pupil  and  friend  of  Galileo,  gave  the  true  ex- 
planation of  the  fact,  and  proved,  as  we  shall  see,  that  this  column  of 
water  of  thirty-two  feet  was  in  equilibrium  with  the  weight  of  the  at- 
mosphere. 

The  celebrated  invention  of  Torricelli  has  sometimes  been  erroneous- 
ly attributed  to  Pascal.  The  French  philosopher  himself  alludes  to  the 
mistake,  and  shows  how  much  of  the  merit  is  due  to  him  in  the  follow- 
ing terms :  "  The  report  of  my  experiments  having  been  spread  abroad 
in  Paris,  they  have  been  confounded  with  those  made  in  Italy ;  and, 
thanks  to  this  misunderstanding,  some,  according  me  an  honor  to  which 
I  can  lay  no  claim,  attributed  the  Italian  experiment  to  me,  while  oth- 
ers unjustly  deprived  me  of  the  credit  of  those  to  which  I  was  really  en- 
titled. To  give  to  others  and  to  myself  the  justice  due  to  us,  I  published, 
in  1647,  the  experiments  which  I  had  made  the  year  before  in  Norman- 


WEIGHT  OF  THE  ATMOSPHERE. 


39 


dy ;  and  that  they  might  not  be  confounded  with  one  made  in  Italy,  1 
gave  the  latter  separately  and  in  italics,  whereas  mine  were  printed  in 
Eoman  letters.  Not  content  with  giving  it  these  distinctive  marks,  I 
have  stated  in  so  many  words  that  I  am  not  the  inventor  of  the  barom- 
eter ;  that  it  was  made  in  Italy  four  years  previously,  and  was  the  cause 
of  my  making  similar  experiments." 

It  was,  then,  the  refusal  of  the  water  to  rise  more  than  thirty-two  feet, 
in  obedience  to  the  pumps,  which  revealed  to  Torricelli  the  fact  that  the 
atmosphere  had  weight,  and  that  its  whole  weight  was  balanced  by  a 
column  of  water  thirty-two  feet  in  height.  Let  us  then  examine  for  a 
moment  the  mechanism  and  action  of  the  pump. 

Every  one  knows  that  these  simple  and  old-fashioned  contrivances 
serve  to  raise  water  either  by  suction  or  pressure,  or  by  both  combined. 
Hence  their  classification  as  suction-pumps,  forcing-pumps,  and  suction 
and  forcing  pumps.  Before  Galileo's  day,  the  ascension  of  water  in  the 
suction-pump  was  ascribed  to  the  fact  of  nature  abhorring  a  vacuum ; 
but  it  is,  in  reality,  merely  an  effect  of  atmospheric  pressure. 

Take  a  tube,  at  the  lower  extremity  of  which  is  a  piston,  and  place 
this  lower  end  in  water.  If  the  piston  is 
drawn  up,  a  vacuum  is  created  below,  and  the 
atmospheric  pressure,  acting  upon  the  surface 
of  the  liquid  external  to  the  pump,  makes  it 
rise  in  the  tube  and  follow  the  movement  of 
the  piston. 

Herein  lies  the  principle  of  the  suction- 
pump,  which  is  essentially  composed  of  the 
body  of  the  pump,  in  which  a  piston  moves, 
communicating  by  a  tube  with  a  reservoir  of 
water  (see  Fig.  4).  At  the  point  where  the 
body  of  the  pump  and  the  suction-tube  join  is 
placed  a  valve,  opening  upward,  and  in  the 
body  of  the  piston  there  is  an  opening  formed 
by  a  similar  valve. 

For  water  to  reach  the  body  of  the  pump, 
the  suction-valve  must  be  less  than  thirty-two 
or  thirty-three  feet  above  the  level  of  the  wa- 
ter in  the  well,  otherwise  the  water  would 
cease  to  rise  at  a  certain  point  in  the  tube,  Fig.  4.-suction-pnmp. 
and  the  motion  of  the  piston  would  be  unable  to  raise  it  any  farther, 


40  TUB  ATMOSPHERE. 

In  addition,  to  insure  raising  at  each  ascent  of  the  piston  a  volume 
of  water  equal  to  the  volume  of  the  body  of  the  pump,  the  spout  must 
be  placed  at  a  less  height  than  thirty-two  feet  above  the  reservoir. 

Thus  the  suction-pump  will  not  raise  water 
to  a  height  of  more  than  thirty-two  feet  ;  but 
the  water  having  once  passed  above  the  pis- 
ton, the  height  to  which  it  can  then  be  raised 
depends  solely  upon  the  force  which  drives 
the  piston. 

The  suction  and  force  pump  (see  Fig.  5) 
raises  water  both  by  suction  and  pressure. 
At  the  base  of  the  body  of  the  pump,  over  the 
orifice  of  the  suction-pipe,  is,  as  before,  a  valve 
opening  upward.  Another  valve,  also  open- 
ing upward,  closes  the  aperture  of  the  bent 
tube,  which  runs  into  a  receptacle  called  the 
air-vessel.*  Then  from  this  reservoir  there 
starts  a  pipe  which  serves  to  raise  the  water 
!.  to  the  required  height.  Finally,  the  farce- 
pump  only  acts  mechanically,  and  does  not 

atmospheric   pressura       ft   differs   on]  y 

from  the  other  in  that  it  has  no  suction-pipe,  its  body  going  right 
into  the  water  which  is  to  be  drawn  up. 

In  reference  to  this  elevation  of  the  water  only  to  a  certain  height, 
Torricelli,  throwing  aside,  like  his  master,  all  idea  of  a  hidden  cause, 
showed-  that  the  pressure  of  the  air  compels  the  water  to  mount  up  into  the 
pipe  from  which  the  air  is  withdrawn,  until  the  weight  of  water  raised 
into  the  pipe  is  equivalent  to  that  of  the  ^ir  which  presses  upon  an 
equal  section  of  the  reservoir  from  which  the  water  is  being  raised.  By 
the  aid  of  this  principle  he  was  led  to  invent  the  barometer.  To  exer- 
cise equal  pressures,  the  liquid  columns  must  be  of  heights  inversely 
proportional  to  their  density.  Thus,  a  liquid  twice  as  heavy  as  water 
would,  with  a  column  of  sixteen  feet,  be  in  equilibrium  with  the  atmos- 
phere; and  quicksilver,  which  is  nearly  thirteen  and  a  half  times  as 
heavy  as  water,  would  be  in  equilibrium  if  the  height  of  the  column 
were  diminished  in  this  proportion  —  that  is,  to  about  twenty-nine  inches. 

*  [The  air-vessel  is  not  essential  to  the  principle  of  the  pnmp  ;  if  it  were  not  used  the  supply 
of  water  would  be  intermittent,  as  in  the  common  suction-pump,  but  the  effect  of  the  elasticity 
of  the  air  in  the  air-vessel  is  to  render  the  stream  of  water  continuous.*—  ED.] 


Fig.  ^.-Suction  and  forcing  pump. 


Pig.  6.— Torricelli  inventing  the  Barometer. 


WEIGHT  OF  THE  ATMOSPHERE. 


43 


This  conclusion  is  easily  verified.     Take  a  glass  tube,  three  feet  in 
length,  and  open  only  at  one  end ;  fill  it  with  quicksilver,  and  then, 

placing  the  finger  on  the  open  end  (see 
Fig.  7),  put  the  lower  portion  of  the 
tube  into  a  basin  filled  with  the  same 
liquid,  with  the  end  closed  by  the  fin- 


rig.  T.— The  tube  full  of  quicksilver. 


Fig.  8.— The  tube  in  the  basin. 


ger  downward.  Immediately  the  finger  is  removed,  the  quicksilver 
inside  will  descend  several  inches  and  then  stop  (see  Fig.  8 ).  The 
equilibrium  is  established,  and  the  liquid  column  which  remains  sus- 
pended in  the  pipe  is  a  true  balance,  for  the  weight  of  the  column  of 
mercury  is  exactly  in  equilibrium  with  the  atmospheric  pressure. 

Torricelli  gave  to  this  tube  of  quicksilver,  thus  placed  vertically  in  a 
basin  of  quicksilver,  the  name  of  Barometer;  that  is  to  say,  a  contriv- 
ance to  indicate  the  weight  of  the  air,  from  the  Greek  flapog,  weight, 
and  fjLirpov,  measure.  Its  invention  by  Torricelli  dates  from  1643. 
Three  years  later,  Pascal  repeated  the  experiment  in  France  with  a  wa- 
ter-barometer, and  even  a  wine-barometer.  This  was  at  Rouen.  His 
tube  was  forty-nine  feet  long,  and  to  avoid  the  difficulty,  insurmounta- 


44  THE  ATMOSPHERE. 

ble  in  that  day,  of  exhausting  the  air  in  it  directly,  he  had  it  sealed  at 
one  end,  filled  it  with  wine,  and  closed  the  other  end  with  a  cork. 
Then,  by  means  of  cords  and  pulleys,  the  tube  was  placed  upright  and 
the  lower  end  put  into  a  vessel  full  of  water.  As  soon  as  the  cork  that 
kept  it  closed  was  removed,  the  whole  liquid  column  in  the  tube  fell, 
until  its  surface  was  about  thirty-three  feet  above  the  level  of  the  water 
in  the  vessel.  The  remaining  sixteen  feet  above  were  destitute  of  air. 
Consequently,  the  liquid  column  itself  formed  an  equilibrium  to  the  at- 
mospheric pressure,  and  from  this  he  drew  the  conclusion  that  a  column 
of  water  (or  of  wine  of  the  same  density)  thirty-two  feet  high  weighs  as 
much  as  a  column  of  air  on  the  same  base. 

The  surface  of  the  earth  is  pressed  upon  as  if  it  was  covered  with 
a  body  of  water  thirty-two  or  thirty-three  feet  deep,  and  we  who  live 
upon  the  bed  of  this  ocean  of  air  undergo  the  same  pressure. 

If  it  is  the  pressure  of  the  air  which  causes  the  elevation  of  the  quick- 
silver or  the  water,  as  we  ascend  into  the  atmosphere,  the  weight  of  the 
column  of  quicksilver  raised,  and  consequently  the  height  of  this  col- 
umn, must  gradually  diminish  in  a  manner  dependent  on  the  strata  of 
air  left  beneath  it.  The  experiment  was  made  on  the  Puy-de-D6me,  ac- 
cording to  the  instructions  of  Pascal,  by  his  brother-in-law,  Florin  Pe- 
rier,  upon  the  19th  of  September,  1648,  and  repeated  by  Pascal  himself 
on  the  Tour  St.  Jacques  at  Paris. .  The  results  were  decisive,  and  the 
barometer  became  an  easy  and  accurate  means  of  measuring  the  total 
weight  of  the  atmosphere,  and  the  variations  in  the  pressure  which  it 
exerts  at  different  times  and  places  upon  the  surface  of  the  globe.  We 
thus  see  that  it  was  between  1643  and  1648  that  the  atmospheric  press- 
ure was  demonstrated  by  the  construction  of  the  barometer  and  the  ex- 
periments which  its  discoverers  at  once  entered  upon. 

By  a  coincidence  not  at  all  unusual  in  the  history  of  science,  while 
the  indications  of  the  barometer  were  being  studied  in  Italy  and 
France,  experiments  were  being  made  in  Holland  to  ascertain  the  pre- 
cise weight  of  the  air,  but  by  quite  a  different  process. 

In  1650,  Otto  de  Guericke,  burgomaster  of  Magdeburg,  invented  the 
air-pump,  by  which  the  air  may  be  exhausted  from  any  receptacle  and 
a  nearly  absolute  vacuum  created. 

The  ingenious  inventor  conceived  in  the  same  year  the  idea  of  weigh- 
ing a  globe  of  glass,  first  leaving  in  it  the  air  which  it  contained,  and 
then  weighing  it  again  when  the  air  had  been  removed  by  the  air- 
pump.  The  globe,  when  emptied  of  air,  was  found  to  be  less  heavy 


WEIGHT  OF  THE  ATMOSPHERE.  45 

by  about  one-third  of  a  grain  for  every  cubic  inch  of  the  globe's  ca- 
pacity. 

Aristotle  had  long  before  suspected  that  air  had  weight,  and  to  make 
sure  of  the  fact,  he  weighed  a  leather  bottle,  first  empty  and  afterward 
when  inflated  with  air;  for,  he  remarked,  if  the  air  has  weight,  the 
leather  bottle  will  be  heavier  when  weighed  the  second  time  than  it 
was  the  first  time.  The  experiment  not  confirming  his  supposition,  he 
concluded  that  the  air  had  no  weight.  Nevertheless,  several  of  the 
ancient  philosophers  admitted  the  material  nature  of  air  as  a  fact. 
Thus  the  Epicureans  compared  the  effects  of  the  wind  with  those  of 
water  in  motion,  and  considered  the  elements  of  the  air  as  invisible 
bodies.  During  the  reign  of  the  peripatetic  philosophy,  however,  it 
was  assumed  that  air  was  without  weight,  and  there  were  but  few 
philosophers  who  did  not  share  this  erroneous  opinion. 

We  have  seen  that,  by  repeating  judiciously  the  experiment  of  Aris- 
totle, Otto  de  Guericke  demonstrated  the  real  weight  of  air.  If  Aris- 
totle's experiment  led  to  a  contrary  result,  it  must  be  attributed  to  the 
change  in  the  volume  of  the  leather  bottle  during  his  two  trials,  for 
every  body,  when  weighed  in  a  fluid,  loses  in  weight  a  quantity  equal 
to  the  weight  of  the  fluid  displaced. 
The  leather  bottle  made  use  of  by  Aris- 
totle would  have  shown  an  increase  of 
weight  if  weighed  in  a  vacuum.  Let 
us  suppose  that  about  1835  cubic  inch- 
es, of  air  were  introduced  into  it  by  in- 
spiration; its  weight  would  have  in- 
creased by  about  550  grains,  but  at  the 
same  time  the  bottle  would  become  in- 
flated, and  its  volume,  being  increased 
by  1835  cubic  inches,  would  have  dis- 
placed a  volume  of  air  of  equal  weight, 
so  that  its  loss  in  weight  would  be  also 
550  grains,  and  the  weight  of  the  air 
and  bottle  together  would  consequent- 
ly remain  the  same  as  before.  But  in 
the  experiment  of  Otto  de  Guericke 
the  globe  was  always  of  the  same  size, 

Whether    empty   Or   full    of  air,  and    its      Fig.  9._Otto  de  Guericke'8  experiment 

loss  in  weight  through  the  displacement  of  the  air  being  in  each  case 


^g  THE  ATMOSPHERE. 

the  same,  there  was,  of  course,  a  difference,  which  proved  that  air  had 
weight.  Otto  de  Guericke,  at  the  same  time,  conceived  the  idea  of  the 
Magdeburg  Hemispheres,  so  called  from  the  town  in  which  he  invented 
them,  and  which  consist  of  two  hollow  hemispheres  of  copper,  with  a 
diameter  of  from  four  to  five  inches.  The  hemispheres  fit  each  other 
hermetically.  One  of  them  has  attached  to  it  a  cock  that  screws  on 
to  the  plate  of  an  air-pump,  and  the  other  a  ring  which  acts  as  a 
handle  to  move  it  backward  or  forward.  As  long  as  the  two  hem- 
ispheres, when  in  contact,  contain  air  within  them  they  can  easily  be 
separated,  for  there  is  equilibrium  between  the  expansive  force  of  the 
interior  air  and  the  outside  pressure  of  the  atmosphere,  but  when 
once  a  vacuum  is  formed  by  the  exhaustion  of  the  air,  it  requires  a 
considerable  effort  to  draw  them  apart. 

In  one  of  these  experiments,  the  learned  burgomaster  had  each 
hemisphere  pulled  by  four  strong  horses  without  succeeding  in  sep- 
arating them.  The  diameter  was  more  than  two  feet,  which  gives  a 
total  of  more  than  three  and  a  quarter  tons  as  the  atmospheric  pressure 
brought  to  bear  in  the  way  of  resistance. 

The  pressure  of  the  atmosphere  on  a  square  inch  is  equivalent  to  the 
weight  of  a  column  of  quicksilver  with 
a  volume  of  29'92  cubic  inches,  viz., 
about  fifteen  pounds. 

It  is  easy  and  interesting  to  draw 
from  this  the  conclusion  that,  as  the 
superficies  of  an  average  human  body 

Fig.  10.-The  Magdeburg  Hemispheres.        ig    sixteen    Q^^Q    ^  W(J    may   each 

of  us  be  said  to  be  subject  to  a  pressure  of  about  fifteen  tons. 

That  we  are  not  crushed  by  this  enormous  pressure,  is  because  it  does 
not  all  press  vertically  down  on  us.  As  the  air  surrounds  us  on  all 
sides,  its  pressure  is  transmitted  over  our  body  in  all  directions,  and, 
in  consequence,  becomes  neutralized.  Air  penetrates  readily  and  with 
full  pressure  into  the  profoundest  cavities  of  our  organism ;  hence  we 
have  the  same  pressure  inside  and  outside,  and  thus  these  weights  be- 
come exactly  balanced.  This  is  easily  proved  by  the  experiment  of 
bursting  a  bladder  under  the  receiver  of  an  air-pump.  Take  a  cylin- 
drical glass  vessel,  hermetically  closed  at  the  upper  end  by  a  piece  of 
gold-beater's  skin,  with  the  other  end  placed  (see  Fig.  11)  on  the  plate 
of  an  air-pump ;  as  soon  as  the  air  begins  to  be  exhausted  from  the 
si,  the  gold-beater's  skin  becomes  depressed  under  the  influence  of 


WEIGHT  OF  THE  ATMOSPHERE.  47 

the  atmospheric  pressure  upon  it  from  above,  and  soon  bursts.  The 
opposite  result  occurs  if  the  pressure  from  out- 
side is  lessened.  If  a  bird  is  placed  in  the  vac- 
uum of  an  air-pump,  its  body  will  be  seen  to 
swell,  its  blood  to  spurt  out  with  violence,  and 
in  a  short  time  it  perishes,  a  victim  to  a  kind  of 
explosion  the  inverse  of  that  just  described. 

This  fact  is  confirmed,  as  we  shall  see  farther 
on,  by  the  ascents  that  have  been  made  to  -great 
elevations.  Upon  reaching  the  regions  where 
the  air  is  much  rarefied,  the  limbs  swell,  and  the 
blood  has  a  tendency  to  force  its  way  through  the 
skin,  in  consequence  of  the  want  of  equilibrium 

Fig.  ii.-Atmospheric  press-  between  its  own  tension  and  that  of  the  external 

ure;  rupture  of  equilibrium.       •     ^ 

Any  one  can  show  the  effect  of  atmospheric  pressure  by  a  very 
simple  experiment.     This  consists  in  filling  a  glass 
quite  with  water  and  laying  over  the  top  a  sheet  of 
paper.     It  can  then  be  turned  over  without  spilling 
any  of  the  liquid,  a  fact  which  must  be  attributed    ^ 
to  the  pressure   which  the  atmosphere  exercises 
upon  the  sheet  of  paper. 

It  was  stated  above  that,  where  a  vacuum  is 
created,  the  atmospheric  pressure  is  about  fifteen 
pounds  to  the  square  inch.  It  is  this  pressure 
which  causes  the  limpet  to  adhere  to  the  rock, 
when  this  mollusk  has  by  contraction  created  a  slass- 
vacuum  under  its  shell.  The  fly,  excluding  the  air  from  between  its 
feet  and  the  ceiling,  is  enabled,  apparently,  to  violate  the  laws  of  grav- 
ity. Cupping-glasses,  when  applied  to  the  body,  act  on  this  same 
principle,  and  we  can  not  take  a  step  without  observing  some  fact 
which  is  founded  on  the  effects  of  atmospheric  pressure.  Such  are 
the  general  facts  and  experiments  which  demonstrated  that  the  air 
had  weight,  and  gave  birth  to  the  instrument  wherewith  this  weight 
was  to  be  determined,  viz.,  the  barometer.  It  now  remains  to  apply 
these  ideas  to  the  whole  atmosphere,  the  extent  of  which  we  endeav- 
ored to  explain  in  the  preceding  chapter. 

*  [I  have  neither  experienced  any  of  these  symptoms  myself,  nor  have  I  observed  them  in 
others. — ED.] 


^g  THE  ATMOSPHERE. 

At  the  level  of  the  sea  the  pressure,  upon  the  average,  sustains  the 
barometrical  column  at  a  height  of  about  29'92  inches. 

Experiments  frequently  repeated  by  physical  philosophers — and  the 
accuracy  of  which  has  been  verified— have  proved  that  the  weight  of 
the  air  at  32°  (Fahr.)  of  temperature,  and  under  a  pressure  of  29'92 
inches  of  mercury,  is  to  the  weight  of  an  equal  volume  of  quicksilver 
in  the  proportion  of  unity  to  10,509 — that  is  to  say,  that  10,509  cubic 
inches  of  air  have  the  same  weight  as  one  cubic  inch  of  mercury.  If 
the  density  of  the  strata  of  air  were  everywhere  the  same,  it  would  be 
easy  to  deduce  from  the  above  result  not  only  the  height  of  a  given 
spot  by  the  aid  of  the  barometer  reading  there,  but  also  the  total  height 
of  the  atmosphere.  It  is,  indeed,  evident  that  if  a  fall  of  an  inch  in  the 
height  of  the  barometer  corresponded  to  a  change  of  height  of  10,509 
inches,  a  fall  of  29-92  inches,  which  is  the  total  height  of  the  barome- 
ter,  would  correspond  to  29'92  times  10,509  inches — that  is,  about  five 
miles.  Such  would  be  the  height  of  the  atmosphere  if  its  density  re- 
mained the  same  from  top  to  bottom,  but  we  have  seen  that  its  lower 
strata  are  denser  than  the  higher.  It  follows,  therefore,  that,  to  pro- 
cure a  fall  of  an  inch  in  the  mercury  of  the  barometer,  it  is  necessary 
to  traverse  a  greater  distance  above  the  level  of  the  ground  or  the  sea. 

Halley  was  the  first  to  deduce  a  formula  by  which  heights  might  be 
obtained  by  means  of  the  barometer. 

We  have  seen  in  the  previous  chapters  that,  since  the  experiments 
of  Mariotte,  it  has  been  recognized  that  air  becomes  compressed  in  pro- 
portion to  the  weight  above,  or  to  the  pressure  exerted  upon  it. 
Thence  it  is  inferred  that,  in  rising  vertically  in  the  atmosphere  to  suc- 
cessive elevations,  increasing  in  arithmetical  progression,  the  density 
of  the  corresponding  strata  of  air  would  diminish  in  geometrical  pro- 
gression. This  would  be  accurate  if  the  temperature  were  everywhere 
the  same,  and  the  difference  in  height  would  scarcely  be  any  more  com- 
plicated than  if  the  density  were  constant.  But  the  temperature  of  the 
air  diminishes  with  increased  height,  so  that  the  variation  in  density  is 
not  so  simple,  as  the  upper  strata  are  more  condensed  by  their  lower 
temperatures  than  those  below. 

The  relation  between  temperature  and  height  is  rather  complicated, 
as  we  shall  see  farther  on ;  and  this,  of  course,  renders  more  difficult 
the  process  of  measuring  heights  by  the  barometer.  At  the  same  time, 
the  atmospheric  strata  always  contain  a  certain  quantity  of  aqueous 
vapor,  the  weight  of  which  must  be  added  to  that  of  the  air. 


WEIGHT  OF  THE  ATMOSPHERE.  49 

Furthermore,  the  weight  of  any  body,  and  consequently  that  of  a 
stratum  of  air,  is  proportionately  less  as  the  body  in  question  is  farther 
removed  from  the  centre  of  the  earth.  And  as  the  weight  of  bodies 
varies  also  according  to  the  latitude  on  account  of  centrifugal  force,  it 
becomes  evident  that,  for  a  single  formula  to  be  in  general  use  for  ob- 
servations made  at  different  points  of  the  globe,  it  is  indispensable  that 
it  should  include  the  latitude  of  the  place  of  observation. 

Laplace  has  given,  in  the  "M6canique  Celeste,"  the  corrections  ren- 
dered necessary  by  these  different  causes  in  measuring  height,  and  has 
deduced  from  theory  alone  a  formula  the  accuracy  of  which  has  been 
confirmed  by  numerous  experiments. 

To  determine  the  height  of  a  mountain  it  is  necessary  that  two  per- 
sons take  simultaneous  observations  of  the  readings  of  the  barometer, 
one  at  its  foot,  the  other  at  its  summit.  They  must  be  careful,  at  the 
same  time,  to  read  the  thermometers  attached  to  the  barometers,  as  well 
as  others  to  determine  the  temperature  of  the  surrounding  air.  Two 
observations  will  be  sufficient ;  but  it  is  better  to  have  several. 

A  single  observer  can  also  ascertain  the  difference  in  level  between 
two  stations,  not  very  distant  the  one  from  the  other,  with  very  fair  ac- 
curacy, if  he  takes  care  to  observe  the  thermometer  and  barometer  at 
the  lower  stations,  both  when  he  leaves  it  and  returns  to  it,  and  infers, 
from  the  difference,  the  reading  at  the  lower  when  taking  that  at  the 
higher  station. 

When,  by  a  long  series  of  observations,  the  average  readings  of  the 
barometer  and  thermometer  at  a  given  place  have  been  determined, 
they  may  be  employed  to  calculate  the  absolute  elevation  of  the  place 
above  the  level  of  the  sea  by  taking  corresponding  observations  at 
the  level  of  the  ocean.  Sufficient  barometrical  observations  have  al- 
ready been  made  at  various  elevations  for  us  to  be  in  a  position  to  rep- 
resent this  decrease  of  atmospheric  pressure,  with  increase  of  elevation, 
no  longer  theoretically,  but  from  direct  observation. 

From  a  series  of  observations,  made  at  very  different  elevations,  the 
table  on  the  following  page  has  been  formed. 

This  satisfactory  series  of  barometrical  observations,  which  we  are 
able  to  establish  by  means  of  numerous  ascents,  either  in  the  balloon 
or  up  the  mountain  path,  and  by  researches  of  several  observers  in 
inhabited  regions  far  above  the  level  of  the  sea,  enables  us  also  to  en- 
deavor to  represent,  by  a  curve  and  a  tint,  this  rapid  decrease  in  the 
weight  of  the  atmosphere.  In  Fig.  13,  the  horizontal  line  which  forms 

4 


THE  ATMOSPHERE. 


.  .  

Height  above 
the  Sea. 

Mean 
Reading. 

Feet. 
0 
159 
213 
472 
650 
804 
1,339 
2,067 
3,937 
4,856 
6,837 
8,130 
8,773 
9,541 
10,893 
13,124 
15,748 
20,014 

22,113 
22,966 
22,966 
26,247 
29,000 
37,000 

Inches. 
29-92 
29-74 
29-68 
29-57 
29-37 
29-21 
28-58 
27-91 
25-98 
25-24 
23-62 
22-17 
21-85 
21-02 
20-08 
18-70 
16-69 
14-17 

13-39 
12-79 
12-60 
10-79 
9-75 
7-00 

MeTn  btrometrilTreading  at  Greenwich  Observatory  

Dijon  (Perrey)  

Geneva  Observatory  (Plantamour)  
TJndpz  (Blondeau}    

Summit  of  Vesuvius  (Palmieri)  

Guatemala  (R.  P.  Canudas)  

Guanaxuato  (Humboldt)  ••  ••  

The  Monastery  of  the  Great  St.  Bernard  

The  Summit  of  the  Faulhorn  (Bi  avals;  

Town  of  Quito  (Fouque')  •  

In  several  aeronautical  ascents  (llammaiion)  

Summit  of  Mont  Blanc  (Ch.  Mai  tins)  \""\(  

The  summit  of  Ibi-Gamin  (the  highest  mountain  that  has  been 

In  the  highest  ascent  (Glaisher)  

the  base  represents  the  mean  state  of  the  barometer  at  the  level  of  the 
sea  (29-92  inches).  Each  other  horizontal  line  indicates  the  reading  of 
the  barometer  'corresponding  to  the  elevation  which  is  shown  by  the 
vertical  line.  In  this  way,  or  by  the  aid  of  the  tinted  portion,  it  will 
be  noticed  that  at  8200  feet  the  pressure  is  diminished  by  one-quarter, 
at  18,000  feet  by  one-half,  and  at  31,168  feet  by  three-quarters. 

The  reading  of  the  barometer  diminishes,  therefore,  rapidly  as  we 
rise  above  the  level  of  the  sea.  But  even  there  it  is  not  the  same  all 
over  the  globe's  surface.  It  is  lower  at  the  equator  than  between  the 
tropics ;  at  the  equator  it  is  about  29'84  inches ;  it  then  increases  up  to 
the  33d  degree  of  latitude,  where  it  is  30'16  inches ;  then  decreases  un- 
til the  43d  degree  (30'00  inches),  toward  which  point  it  becomes  sta- 
tionary, and  so  remains  up  to  the  forty-eighth  degree.  Thence  it  con- 
tinues to  decrease  so  far  as  sixty-four  degrees,  where  it  stands  at  29'65 
inches.  Lastly,  it  again  increases  from  that  point  as  far  as  the  remotest 
latitudes — at  Spitzbergen  (seventy-fifth  degree),  where  the  height  of  the 
barometer  is  29-84  inches.  Between  the  pressures  at  the  thirty-third 
degree  and  the  sixty-fourth  degree  of  latitude,  tliere  is,  therefore,  a  dif- 
ference of  half  an  inch.  I  have  laid  down  these  results  on  a  diagram, 
and  traced  the  following  curve  (see  Fig.  14,  p.  52): 

These  variations  in  the  atmospheric  pressure  are  probably  caused  by 


WEIGHT  OF  THE  ATMOSPHERE. 


51 


the  trade-winds  and  upper  currents  of  air,  which  slightly  raise  the  whole 
mass  of  the  atmosphere. 

It  is  easy  to  conceive  that  the  latitude  may  exercise  some  influence 
upon  the  pressure  of  the  air,  inasmuch  as  the  conditions  of  temperature, 
pressure,  and  rotary  movement  vary  with  it.  It  is  less,  easy  to  explain 


<r  "49  I 


tax;  A A.jftrp.JtK?: 

JS.SZ 


Fig.  13.— Diagram  showing  the  decrease  of  atmospheric  pressure,  according  to  height 

why  the  longitude  should  exercise  any,  but  it  seems,  nevertheless,  to  do 
so.  In  the  same  latitude,  the  average  pressure  of  the  atmosphere  is 
O14  inch  greater  in  the  Atlantic  than  in  the  Pacific  Ocean. 

The  readings  of  the  barometer  are  continually  changing;  but,  not- 
withstanding this,  by  a  careful  determination  of  the  mean  atmospheric 


52  THE  ATMOSPHERE. 

pressure  at  many  places,  a  map  showing  the  lines  of  equal  barometrical 
pressure  (isobaric  lines)  can  be  drawn  over  the  surface  of  our  planet 

The  lines  of  equal  pressure-^r  isobaric  lines,  as  they  are  technically 
termed-*!*  at  first  pretty  equally  distributed  from  N.  to  3,  running 
from  WSW  to  E.N.E.  The  isobaric  line  of  29'96  inches  passes 
through  ihe  south  of  England  and  Holland;  that  of  30'02  inches  near 
Tours°and  Nancy;  but  the  centre  of  France  shows  a  very  remarkable 


of  23.5  in  he 


30  00 

29.93 


Latitude  0  5         10        15        20         25        30        35        *0        45        .50        55         60        65         70         75        60 

Pig.  14.— Variation  in  the  atmospheric  pressure  at  the  level  of  the  sea,  from  the  Equator  to  the  North 

Pole. 

line  of  pressure,  for  the  isobaric  line  of  30-04  inches  crosses  France 
diagonally,  passing  close  to  Strasburg,  Chaumont,  Dijon,  Clermont,  and 
Toulouse.  On  the  other  side,  toward  the  S.E.,  the  pressure  diminishes, 
and  attains  a  minimum  not  less  remarkable  in  the  Gulf  of  Grenoa,  where 
the  pressure  is  about  29 '98  inches. 

The  curve  of  30'00  inches  is  formed,  and  its  path  pretty  well  known, 
in  consequence  of  the  numerous  points  at  which  observations  have  been 


WEIGHT  OF  THE  ATMOSPHERE.  53 

made.  The  isobaric  line  of  3O08  inches,  which  passes  close  to  Oran, 
and  somewhat  farther  from  Algiers,  necessarily  continues  toward  the 
west,  nearly  parallel  with  the  above.  A  maximum  of  pressure  in  the 
Atlantic  is  in  thirty -five  degrees  of  north  latitude;  a  minimum  of 
pressure  is  met  with  at  five  degrees  north  of  the  equator ;  a  maximum 
at  sixteen  degrees  south  latitude,  near  St.  Helena;  and  the  lowest 
pressure  existing  in  the  world  is  to  the  south  of  Cape  Horn,  where  it 
does  not  exceed  29 '33  inches.  Upon  the  Asiatic  continent  the  distribu- 
tion is  quite  different,  and  Siberia  shows  a  maximum  of  about  3O24 
inches  between  Nertchinsk  and  Bernaoul. 

The  chief  difficulty  in  calculating  altitudes  is  in  reference  to  the 
mean  level  of  the  sea.  Equilibrium  upon  the  surface  of  the  sea  is  not 
absolute ;  its  level  is  affected  by  various  causes,  such  as  centrifugal 
force  in  the  zone  of  the  equator,  the  wind,  barometrical  pressure,  and 
temperature.  To  these  may  be  added  the  configuration  of  the  sea- 
board, which  gives  a  varying  effect  to  the  action  of  the  winds  and 
tides.  It  is  well  known  that  the  sea  rises  quicker  than  it  recedes,  and 
when  the  gulfs  are  landlocked  this  effect  is  more  decided.  Along  the 
coast  the  sea  must  rise  higher  than  it  does  farther  from  shore. 

The  level  of  the  sea  at  Marseilles  is  31 '5  inches  lower  than  the  aver- 
age level  of  the  ocean  upon  the  French  coast.  The  Mediterranean  must 
be  an  inclined  plane,  falling  from  the  Straits  of  Gibraltar  to  the  coast  of 
Syria.  The  last  level  taken  in  Egypt,  from  the  Mediterranean  to  the 
Eed  Sea,  showed  that  the  latter  is  higher  than  the  Mediterranean.  It  is 
easy  to  comprehend  that  these  seas,  receiving  much  less  water  than 
evaporates  from  them,  must  have  a  tendency  to  become  shallow,  and 
that  they  are  only  kept  up  by  the  straits  that  unite  them  with  the  ocean. 

This  first  general  description  of  the  weight  of  the  air  and  its  pressure 
upon  the  spherical  surface  of  the  globe  will  answer  our  present  purpose. 
It  explains  in  some  degree  the  statics ;  and  we  shall  soon  reach  the  dy- 
namics. The  atmosphere  is  unceasingly  in  motion,  with  its  displace- 
ments, horizontal,  vertical,  and  oblique.  From  this  cause  it  results  that 
the  weight  of  air  upon  a  given  place,  or  the  height  of  the  barometer,  is 
always  changing.  Solar  heat  gives  rise  to  regular  diurnal  and  monthly 
variations,  the  intensity  of  which  differs  according  to  the  latitude.  The 
change  in  the  position  of  the  great  currents  gives  rise  to  extensive  vari- 
ations upon  a  vast  scale.  Changes  of  weather  are  heralded  by  these 
fluctuations,  which  are  bound  up  with  the  general  pressure. 

Under  the  title  of  "Combienpese  la  masse  entire  de  tout  Tair  qui  est  au 


54  THE  ATMOSPHERE. 

Monde"  Pascal  wrote,  at  the  epoch  when  he  devoted  himself  to  his  cele- 
brated experiments  on  atmospheric  pressure,  a  small  treatise  as  simple 
as  it  is  curious,  the  first  sketch  of  all  that  has  since  been  written  on  this 
subject,  and  containing  from  the  outset  the  absolute  reply  to  the  ques- 
tion which  forms  its  title.  "We  learn,"  he  says,  "by  these  experi- 
ments that  the  air  which  is  over  the  sea-level  weighs  as  much  as  water 
to  a  height  of  thirty-two  feet;  but  inasmuch  as  the  air  weighs  less  over 
more  elevated  places,  and  consequently  does  not  press  equally  over  all 
points  of  the  earth  alike,  it  is  impossible  to  measure  exactly  what  is  the 
pressure  upon  all  parts  of  the  world  by  the  same  process,  although  an 
approximate  measure,  very  nearly  accurate,  may  be  taken.  Thus,  for 
instance,  it  may  be  assumed  that  all  the  places  of  the  earth  have  as  much 
pressure  upon  them  as  if  there  was  a  depth  of  rather  more  than  thirty- 
two  feet  of  water  over  them ;  and  it  is  certain  that  this  supposition  is 
not  half  a  foot  in  error. 

"  Now  we  have  seen  that  air  which  is  above  the  mountains  3000  feet 
high  is  as  heavy  as  water  to  a  height  of  twenty-nine  feet.  Consequent- 
ly, all  the  air  which  extends  from  the  level  of  the  sea  to  the  summit  of 
the  mountains  weighs  nearly  the  seventh  part  of  the  whole  atmosphere. 

"We  gather,  too,  from  this,  that  if  the  whole  sphere  of  the  air  was 
compressed  against  the  earth  by  a  force  which,  driving  it  downward,  re- 
duced it  to  so  small  a  space  that  it  became  of  the  density  of  water,  it 
would  then  be  only  thirty-two  feet  high.  The  whole  mass  of  air  may 
be  regarded  as  if  it  had  been  formerly  a  mass  of  water,  thirty-two  feet 
deep,  which  had  become  rarefied  and  very  much  dilated,  and  converted 
into  the  state  which  we  call  air;  whereas  it  occupies,  in  truth,  more 
space,  though  it  preserves  exactly  the  same  weight.  t 

"And  as  nothing  would  be  simpler  than  to  calculate  what  would  be 
the  weight  in  pounds  of  water  surrounding  the  earth  to  ft  depth  of  thir- 
ty-two feet,  we  should  find,  by  the  same  means,  the  weight  of  the  entire 
mass  of  air. 

"  Curiousity  led  me  to  make  this  calculation,  and  I  found  that  the 
weight  of  this  mass  of  water  would  be  about  nine  trillions  of  pounds 
— that  is,  nine  followed  by  eighteen  ciphers  represents  the  weight,  in 
pounds,  of  air  surrounding  the  earth." 

This  weight  is  about  iio;00)>  part  of  the  weight  of  the  earth. 

If  all  this  mass  of  air  were  agglomerated  into  a  single  ball,  it  would 
weigh  as  much  as  a  ball  of  copper  with  a  diameter  of  sixty-two  miles. 
Thus  the  weight  of  the  air  is  far  from  being  insignificant. 


Fig.  15.— Lavoisier  analyzing  Atmospheric  Air. 


CHEMICAL  COMPONENTS  OF  THE  AI2t.  57 


CHAPTER  Y. 

CHEMICAL  COMPONENTS  OF  THE  AIR. 

IT  is  to  the  great  French  chemist  Lavoisier  that  science  owes  the  dis- 
covery of  the  chemical  components  of  the  air.- 

Let  us  go  back  to  the  researches  of  this  laborious  observer,  and  hear 
from  his  own  lips  the  recapitulation  of  his  interesting  studies. 

Our  atmosphere,  he  remarks,  must  be  made  up  of  all  the  substances 
capable  of  remaining  in  an  aeriform  state  at  the  ordinary  degree  of  tem- 
perature and  atmospheric  pressure  which  we  experience.  These  fluids 
form  a  mass,  almost  homogeneous,*  from  the  surface  of  the  earth  to  the 
highest  elevation  which  man  has  ever  reached,  and  the  density  of  which 
decreases  with  elevation.  But  it  is  possible  that  above  our  atmosphere 
there  are  several  strata  of  very  different  fluids. 

What  is  the  number,  and  what  is  the  nature,  of  the  elastic  fluids 
which  compose  this  lower  stratum  that  we  inhabit? 

After  having  established  the  fact  that  chemistry  offers  two  methods 
essential  for  the  study  of  bodies — that  is  to  say,  analysis  and  synthesis — 
Lavoisier  describes  as  follows  the  celebrated  experiment  of  the  first 
analysis  of  air : 

"  Taking  a  vessel,  or  long-necked  tube,  with  a  bell  or  globe  at  its  ex- 
tremity, containing  about  thirty-six  cubic  inches  (see  Fig.  16,  p.  58),  I  bent 
it  (see  Fig.  17,  p.  58)  so  as  to  place  it  in  the  furnace  while  the  extreme  end 
of  the  neck  was  under  a  glass  cover,  which  was  placed  in  a  basin  of  mer- 
cury. Into  this  vessel  I  poured  four  ounces  of  very  pure  mercury ;  and 
then,  by  means  of  a  siphon,  I  raised  tne  mercury  to  about  three-quarters 
the  height  of  the  glass  cover,  and  marked  the  level  by  gumming  on  a 
strip  of  paper.  I  then  lighted  the  fire  in  the  furnace,  and  kept  it  up  in- 
cessantly for  twelve  days,  the  mercury  being  just  sufficiently  heated  to 
boil.  At  the  expiration  of  the  second  day,  small  red  particles  formed 
upon  the  surface  of  the  mercury,  and  increased  in  size  and  number  for 
the  next  four  or  five  days,  when  they  became  stationary.  At  the  end 

*  [Homogeneous  must  be  understood  to  mean  that  the  components  of  the  atmosphere  are 
found  mixed  in  the  same  proportion  at  all  heights.  Its  usual  meaning  is,  of  course,  "of  uni- 
form density." — ED.] 


THE  ATMOSPHERE. 


of  the  twelve  days,  seeing  that  the  calcination  of  the  mercury  made  no 
further  progress,  I  let  out  the  fire  and  set  the  vessels  to  cool.  The  vol- 
ume of  air  contained  in  the  body  and  neck  of  the  vessel  before  the  op- 
eration was  fifty  cubic  inches;  and  this  was  reduced  by  evaporation  to 
forty-two  or  forty-three.  On  the  other  hand,  I  found,  upon  carefully 
collecting  the  red  particles  out  of  the  melted  mercury,  that  their  weight 
was  about  forty-five  grains.  The  air  which  remained  after  this  opera- 
tion, and  which  had  lost  a  sixth  of  its  volume  by  the  calcination  of  the 
mercury,  was  no  longer  fit  for  respiration  or  combustion,  as  animals 
placed  in  it  died  at  once,  and  a  candle  was  extinguished  as  if  it  had  been 
plunged  in  water.  Taking  the  forty-five  grains  of  red  particles,  and 
placing  them  in  a  small  glass  vessel,  to  which  was  adapted  an  apparatus 
for  receiving  the  liquids  and  aeriform  bodies  which  might  become  sepa- 


Pig.  16.— The  glass  vessel.  Fig.  17.— The  apparatus. 

rated,  and  having  lighted  the  fire  in  the  furnace,  I  observed  that  the 
more  the  red  matter  became  heated,  the  deeper  became  its  color.  When 
the  vessel  approached  incandescence,  the  red  matter  commenced  to  be- 
come smaller,  and  in  a  few  minutes  had  quite  disappeared ;  and  at  the 
same  time  forty-one  and  a  half  grains  of  mercury  became  condensed  in 
the  small  receiver,  and  from  seven  to  eight  cubic  inches  of  an  elastic 
fluid,  better  adapted  than  the  air  of  the  atmosphere  to  supply  the  respi- 
ration of  animals  and  combustion,  passed  under  the  glass  cover.  From 
the  consideration  of  this  experiment,  we  see  that  the  mercury,  while  it 
is  being  calcined,  absorbs  the  only  portion  of  the  air  fit  for  respiration, 
or,  to  speak  more  correctly,  the  base  of  this  portion  ;  and  the  rest  of  the 
air  which  remains  is  unable  to  support  combustion  or  undergo  respira- 
tion. Atmospheric  air  is,  therefore,  composed  of  two  elastic  fluids  of 
different,  and  even  opposite,  natures." 


CHEMICAL  COMPONENTS  OF  THE  AIR.  59 

The  nature  of  air  was  thus  clearly  established  by  these  experiments, 
which  were  made  in  1777.  Its  real  components  were  not,  however, 
completely  ascertained  until  the  present  century.  The  first  exact  analy- 
sis of  air  is  scarcely  fifty  years  old,  and  is  due  to  Gay-Lussac  and  Hum- 
boldt,  who  analyzed  it  by  the  use  of  the  eudiometer. 


Fig.  18.— Mercury-Eudiometer,  for  analyzing  air. 

When  an  equal  mixture  of  air  and  pure  hydrogen  are  set  fire  to  in 
the  eudiometer,  all  the  oxygen  disappears  in  the  shape  of  water,  which 
becomes  condensed  into  dew,  the  volume  of  which  is  insensible,  and 
there  remains  a  mixture  formed  of  nitrogen  and  the  excess  of  hydro- 
gen employed.  Now  the  hydrogen  causes  a  volume  of  oxygen  equal 
to  half  itself  to  disappear  as  water;  whence  it  follows  that  the  volume 
of  oxygen  contained  in  the  measured  air  is  equal  to  one-third  of  the 
volume  that  has  disappeared.  If  the  measures  of  the  air,  the  hydro- 
gen, and  the  gases  after  explosion,  are  made  at  the  same  pressure  and 
the  same  temperature,  and  if,  in  addition,  the  gases  were  saturated 
with  humidity  before  explosion,  the  determination  would  require  no 
correction.  Such  is  the  principle  of  the  method.  Gay-Lussac  and 
Humboldt  found  that  there  was  twenty-one  per  cent,  of  oxygen,  and 
seventy-nine  per  cent,  of  nitrogen,  in  the  air.  This  analysis  has  since 
been  confirmed  by  nearly  all  chemists.  There  is  another  method  by 
means  of  which  the  relative  quantities  of  oxygen  and  nitrogen  con- 
tained in  the  air  of  the  atmosphere  can  be  weighed — a  process  which 
gives  results  far  more  accurate  than  the  measuring  of  the  volumes  (al- 


60  THE  ATMOSPHERE. 

ways  very  small)  of  the  gases  employed  in  the  other  processes.  The 
apparatus  used  is  composed-first,  of  a  tube  which  brings  m  the  air 
from  outside  of  the  room  where  the  operation  is  proceeding;  secondly, 
of  a  set  of  Liebig  balls,  L,  containing  a  concentrated  solution  of  caustic 
potash;  thirdly,  of  a  tube,/  in  the  shape  of  the  letter  U  several  times 
repeated,  and  filled  with  fragments  of  caustic  potash;  fourthly,  of  a 
second  set  of  balls,  o,  containing  concentrated  sulphuric  acid;  fifthly, 
of  a  second  tube,  Z,  of  the  same  shape  as  the  one  above  mentioned, 
filled  with  pumice-stone  steeped  in  concentrated  sulphuric  acid;  sixth- 
ly, of  a  straight  tube,  T,  of  hard  glass.  This  tube  is  filled  with  copper 
filings,  and  laid  upon  a  long  iron  furnace,  so  that  it  can  be  heated 


Fig.  19.— Apparatus  for  analyzing  air  by  the  method  of  weight 

throughout  its  whole  length,  and  is  moreover  furnished  at  its  extremi- 
ties with  two  taps,  r  and  r',  which  admit  of  its  being  emptied ;  seventh- 
ly, of  a  glass  globe,  B,  holding  from  two  to  three  gallons,  and  the  neck 
of  which  is  fitted  with  a  tap,  R. 

To  perform  the  experiment,  as  complete  a  vacuum  as  possible  is 
made  in  the  tube  T  ;  the  two  taps  are  closed  tight,  and  the  tube,  thus 
emptied  of  air,  is  weighed.  The  glass  ball  B,  having  been  emptied  of 
air,  is  also  weighed.  The  various  portions  are  then  put  together  in 
the  order  described,  and  the  tube  T  is  made  red-hot.  Then  the  taps 
r  r'  of  the  tube  T,  and  the  tap  R  of  the  glass  ball,  are  successively 
opened.  The  air,  entering  by  the  suction-tube  to  the  right,  traverses 
first  of  all  the  balls  L  and  the  tube  /,  where  it  parts  with  its  carbonic 
acid ;  then  it  passes  into  the  second  set  of  balls,  o,  and  into  the  tube  Z, 


CHEMICAL  COMPONENTS  OF  THE  AIR.  61 

where  the  sulphuric  acid  removes  all  the  vapor  of  water  it  contains. 
Separated  from  these,  the  air  makes  its  way  into  the  tube  T,  containing 
the  red-hot  copper,  which  retains  the  oxygen,  and  then  passes  into  the 
empty  glass  ball  in  a  state  of  pure  nitrogen.  The  increase  of  weight 
in  the  tube  clearly  gives  the  weight  of  the  oxygen  which  has  been  de- 
posited in  the  operation.  The  difference  between  the  weight  of  the 
globe  when  empty  and  when  full  of  nitrogen  as  clearly  represents  the 
weight  of  this  gas.  By  means  of  this  analysis,  made  with  every  con- 
ceivable precaution,  MM.  Dumas  and  Boussingault  ascertained  that 
one  hundred  parts  of  air  contain — 

Oxygen,  23  in  weight;  20 '8  in  volume. 
Nitrogen,  77          "         79 '2        " 

The  difference  between  the  proportion  of  weight  and  that  of  volume 
is  due  to  the  fact  that  oxygen  is  rather  heavier  than  nitrogen. 

These,  therefore,  are  the  two  fundamental  elements  of  the  chemical 
constitution  of  air.  But  there 
exist  other  elements  in  far 
smaller  quantities;  such,  for 
instance,  as  carbonic  acid  and 
aqueous  vapor.  Their  quan- 
tity is  determined  by  the  ap- 
paratus described  for  finding 
the  weight  of  the  oxygen  and 
nitrogen  in  the  air.  (See  Fig. 
20.)  An  iron  vessel  is  filled 

with   water,  and   emptied   by  \   uSI^t--  "k- 

means  of  a  tap  inserted  in  the 
lower  part  The  water  which 
runs  out  is  gradually  replaced 

0  J         r  Fig  20 — Apparatus  for  obtaining  the  proportion  of  car- 

by  external  air,  which  has  to  bonic  acid  in  air. 

pass  through  the  six  curved  tubes  before  it  reaches  the  reservoir.  The 
first  two  of  these  are  filled  with  pumice-stone  steeped  in  sulphuric 
acid,  and  the  air,  on  its  way  through  them,  leaves  behind  the  water 
which  was  mixed  with  it.  The  two  middle  tubes  are  filled  with  a 
concentrated  solution  of  potash,  which  absorbs  the  carbonic  acid.  Of 
the  last  two  tubes,  containing  pumice-stone  steeped  in  sulphuric  acid, 
the  first  is  intended  to  extract  the  humidity  which  the  potash  has  im- 
parted to  the  air,  and  the  other  to  prevent  the  humidity  from  making 


g2  THE  ATMOSPHERE. 

its  way  back  from  the  sucker  into  the  tubes.  By  weighing,  before 
and  after  the  experiment,  the  series  of  analyzing  tubes,  we  obtain  the 
weight  of  the  water  and  the  weight  of  the  carbonic  acid  contained  in  a 
volume  of  air  equal  to  that  of  the  reservoir. 

The  atmosphere  contains  about  -^^  of  its  volume  of  carbonic  acid. 
There  is  also  a  very  simple  process  by  which  the  oxygen  and  the 

nitrogen  can  be  separated.  Into  a 
graduated  tube,  containing  a  certain 
volume  of  air,  with  its  open  end 
placed  in  a  vessel  containing  water 
or  mercury,  is  inserted  a  long  stick 
of  phosphorus.  (Fig.  21.)  At  the 
expiration  of  six  or  seven  hours,  as 
a  rule,  the  oxygen  is  absorbed,  and 
the  stick  of  phosphorus  may  be 
withdrawn,  and  the  gas  which  re- 
mains— that  is  to  say,  the  nitrogen 
— measured.  The  absorption  is  con- 
sidered to  be  complete  (the  appara- 

Fig.  21._Apparatn8  for  separating  the  oxygen  tUS  being  placed    ill    the   dark)  when 

there  ceases  to  be  any  glimmer  upon 

the  surface  "of  the  phosphorus.  The  rapid  absorption  of  the  oxygen 
by  the  phosphorus  may  be  shown  by  .heating  the  gas  in  a  bell-glass 
into  which  a  fragment  of  phosphorus  has  been  introduced ;  the  phos- 
phorus is  heated  by  an  alcohol-lamp,  and  a  portion  of  it  volatilized ; 
and  when  the  flame  has  reached  all  the  space  occupied  by  the  gas,  the 
experiment  is  complete.  Time  is  left  for  it  to  get  cool ;  .the  volume 
of  nitrogen  is  transferred  into  a  graduated  tube  and  measured,  the  dif- 
ference from  the  original  weight  giving  the  quantity  of. oxygen. 

Oxygen  and  nitrogen  are  two  permanent  Bases'—that  is-'to  say,  it  has 
been  found  impossible  hitherto,  either  by  compression  or  cold,  to  de- 
stroy their  gaseous  form. 

The  first,  oxygen,  is  the  ordinary  agent  of  combustion,  whether  of 
the  kind  which  takes  place  in  our  fire-places  or  in  our  organisms. 
The  second,  nitrogen,  exercises  a  moderating  influence  over  the  first 

Carbonic  acid,  which  exists  in  quantities  varying  according  to  time 
and  place,  but  always  very  small  in  amount,  has  been  liquefied  under 
a  strong  pressure  conjoined  to  intense  cold ;  it  has  even  been  solidified. 
In  that  state  it  has  the  appearance  of  light  and  very  compressible  snow, 


CHEMICAL  COMPONENTS  OF  THE  AIR.  63 

the  contact  of  which  with  the  skin  produces  a  burning  sensation,  this 
excessive  cold  acting  upon  the  epidermis  in  the  same  way  as  great 
heat.*  In  the  small  quantities  in  which  it  is  found,  carbonic  acid  pro- 
duces no  ill  effects ;  in  larger  quantities  it  is  hurtful  to  the  breathing, 
and  finally  produces  asphyxia. 

Emanations  from  the  earth,  the  abundant  sources  of  carbonic  acid, 
are  often  met  with  in  volcanic  districts.  When  M.  Boussingault  ex- 
plored the  craters  at  the  equator,  he  was  shown  a  locality  where  no 
animals  could  remain ;  this  was  at  Tunguravilla,  not  far  from  the  vol- 
cano of  Tunguragua.  He  thus  describes  his  visit  of  1851 :  "  Our  horses 
soon  gave  us  indications  that  we  were  approaching  it ;  they  refused  to 
obey  the  spur,  and  threw  up  their  heads  in  a  most  disagreeable  fashion. 
The  ground  was  strewn  with  dead  birds,  among  which  was  a  magnifi- 
cent black-cock,  that  our  guides  at  once  picked  up.  Among  the  vic- 
tims were  also  several  reptiles  and  a  multitude  of  butterflies.  The 
sport  was  good,  and  the  game  did  not  seem  too  high.  An  old  In- 
dian, Quichua,  who  accompanied  us,  declared  that,  to  procure  a  good 
sleep,  there  was  nothing  like  making  one's  bed  upon  the  Tungura- 
villa." 

This  deleterious  emanation  made  itself  manifest  by  the  sterility  of 
the  ground  for  a  circle  of  some  hundred  yards ;  it  was  especially  great 
at  a  point  where  there  were  many  large  trees  lying  dried  up  and  half 
buried  in  the  vegetable  earth,  which  implies  that  these  trees  had  flour- 
ished upon  the  spot  where  they  have  been  lying  since  the  eruption  of 
the  carbonic  acid.  This  gas,  like  that  which  is  also  met  with  in  similar 
circumstances  in  various  regions  of  the  globe,  is  carbonic  acid  more  or 
less  mixed  with  air,  according  to  its  distance  above  the  soil. 

Carbonic  acid  exercises  a  directly  deleterious  effect  upon  the  nerves 
and  brain.  Hence  the  anaesthetic  effects  which  it  may  produce,  and 
which  all  visitors  to.Pouzzoles,  near  Naples,  may  have  seen  at  a  grotto 
which  has  become  famous  from  this  cause. 

The  keeper  has  a  dog  whose  legs  he  ties  together,  to  prevent  his 
runhing  away ;  he  then  places  him  in  the  middle  of  the  grotto.  The 
animal  displays  evident  fear,  struggles  to  escape,  and  soon  appears  to 
be  dying.  His  master  then  takes  him  out  into  the  open  air,  where  he 
gradually  recovers  himself.  One'of  these  dogs  has  been  used  for  this 
purpose  more  than  three  years.  It  is  all  but  proved  now  that  the  con- 

*  [The  snow-like  flakes  can  be  handled  with  impunity ;  it  is  only  when  forcibly  pressed 
against  the  skin  that  a  blister  is  produced. — ED.] 


64  THE  ATMOSPHERE. 

vulsions  of  the  pythonesses  charged  with  expounding  the  decrees  of 
the  gods  were  produced  by  the  priests  with  carbonic  gas. 

This  grotto  is  situated  upon  the  slope  of  a  very  fertile  hill,  opposite, 
and  not  far  from,  Lake  Agnano.  The  entrance  is  closed  by  a  gate  of 
which  the  keeper  retains  the  key.  It  has  the  appearance  and  shape  of 
a  small  cell  the  walls  and  vault  of  which  have  been  rudely  cut  in  the 
rock.  It  is  about  one  yard  wide,  three  deep,  and  one  and  a  half  high, 
and  it  is  difficult  to  judge  from  its  aspect  whether  it  is  the  work  of  man 
or  of  nature.  The  ground  in  this  cavern  is  very  earthy,  damp,  black, 
and  at  times  heated.  It  is,  as  it  were,  steeped  in  a  whitish  mist,  in 
which  can  be  distinguished  small  bubbles.  This  mist  is  composed  of 
carbonic  acid  gas,  which  is  colored  by  a  small  quantity  of  aqueous 
vapor.  The  stratum  of  gas  is  from  ten  to  twenty-five  inches  high.  It 
represents,  therefore,  an  inclined  plane  the  highest  part  of  which  cor- 
responds to  the  deepest  portion  of  the  grotto,  and  this  is  a  physical 
consequence  of  the  formation  of  the  ground.  The  grotto  being  about 
on  the  same  level  as  the  opening  leading  into  it,  the  gas  finds  its  way 
out  at  the  door,  and  flows  like  a  rivulet  along  the  hill-path.  The 
stream  may  be  traced  for  a  long  distance,  and  a  candle  dipped  into  it 
at  a  distance  of  more  than  six  or  seven  feet  from  the  grotto  is  extin- 
guished at  once.  A  dog  dies  in  the  grotto  in  three  minutes,  a  cat  in 
four,  a  rabbit  in  seventy-five  seconds.  A  man  could  not  live  more 
than  ten  minutes  if  he  were  to  lie  down  upon  this  fatal  ground.  It  is 
said  that  the  Emperor  Tiberius  had  two  slaves  chained  up  there,  and 
that  they  perished  at  once ;  and  that  Peter  of  Toledo,  Viceroy  of  Na- 
ples, shut  up  in  the  grotto  two  men  condemned  to  death,  whose  end 
was  as  rapid. 

Two  analyses  of  the  air  in  this  grotto,  which  had  been  collected  at 
different  times  (see  Ch.  Ste.  01.  Devi  lie  and  F.  Le  Blanc),  gave  in  vol- 
ume— 

Carbonic  acid 67'1  73'6 

Oxygen 6'5  5'3 

Nitrogen 26'4  21-1 

lOO'O          100-0 

It  is  not  necessary  to  travel  so  far  for  this  predominance  of  carbonic 
acid.  At  Montrouge,  near  Paris,  and  in  the  neighborhood,  there  are 
large  quarries,  and  even  cellars,  which  are  filled  from  time  to  time  with 
this  mephitic  gas. 

Upon  the  borders  of  Lake  Laacher,  near  the  Rhine,  and  at  Aigue- 


CHEMICAL  COMPONENTS  OF  THE  AIR.  65 

perse,  in  Auvergne,  there  are  two  sources  of  carbonic  acid  so  abundant 
that  they  give  rise  to  accidents  in  the  open  country.  The  gas  rises  out 
of  small  hollows  in  the  ground,  where  the  vegetation  is  very  rich ;  the 
insects  and  small  animals,  attracted  by  the  richness  of  the  verdure,  seek 
shelter  there,  and  are  at  once  asphyxiated.  Their  bodies  attract  the 
birds,  which  also  perish. 

In  former  times  the  accidents  caused  by  this  gas  in  caves,  mines,  and 
even  in  wells,  gave  rise  to  the  most  extravagant  stories.  Such  locali- 
ties were  said  to  be  haunted  by  demons,  gnomes,  or  genii,  the  guardians 
of  subterranean  treasures,  whose  glance  alone  caused  death,  as  no  trace 
of  lesion  or  bruise  was  to  be  found  on  the  unfortunate  persons  so  sud- 
denly struck  down. 

In  addition  to  the  oxygen,  nitrogen,  and  carbonic  acid,  the  air  con- 
tains a  certain  number  of  other  substances,  in  smaller  and  very  varying 
quantities. 

The  most  important  is  aqueous  vapor,  of  which  I  have  spoken  above 
in  describing  the  method  of  analysis  for  determining  its  presence.  The 
air  always  contains  a  certain  proportion  of  aqueous  vapor  in  a  state  of 
solution,  and  invisible.  When  this  water  passes  into  the  state  termed 
vesicular,  it  constitutes  clouds  or  mists.  The  quantity  of  aqueous  vapor 
varies  with  the  seasons,  the  temperature,  the  altitude,  the  geographical 
position,  etc.  At  the  same  temperature  and  under  the.  same  pressure 
the  maximum  quantity  capable  of  being  mixed  with  the  air  is  invaria- 
ble. The  hygrometrical  state  of  the  air,  for  a  given  temperature,  is  but 
the  relation  between  the  quantity  of  moisture  really  existing  in  the  air 
and  the  quantity  which  would  exist  if  the  air  were  saturated  at  the  same 
temperature.  The  millions  of  cubic  feet  of  vapor  of  water  which,  mix- 
ing with  the  air,  form  the  clouds  and  the  rain  constitute  the  most  im- 
portant element  of  the  atmosphere  in  respect  to  the  circulation  of  life. 
Therefore  water  will  be  in  a  subsequent  chapter  the  object  of  special 
study.  The  quantity  of  heat  necessary  for  the  evaporation  of  the  water 
from  the  earth's  surface  has  been  ascertained.  The  volume  annually 
evaporated  may  be  represented  by  the  volume  of  water  which  falls  from 
the  atmosphere  in  that  space  of  time ;  and,  in  comparing  the  results  of 
observations  taken  at  different  latitudes  and  in  both  hemispheres,  we 
are  led  to  estimate  this  volume  as  corresponding  to  a  depth  of  fifty-four 
and  a  quarter  inches  over  the  whole  earth.  The  amount  of  heat  neces- 
sary to  evaporate  such  a  volume  of  water  would  suffice,  according  to 
Daubree,  to  liquefy  a  thickness  of  ice  of  nearly  thirty -three  feet  in  depth 

5 


QQ  THE  ATMOSPHERE. 

enveloping  the  whole  globe.  From  the  calculations  of  Dalton,  the  at- 
mosphere contains  about  the  0'0142th  part  of  its  weight  in  water:  the 
upper  strata  are  nearly  free  from  water. 

What  other  .substances  are  there  to  be  found  in  the  atmosphere?  It 
unquestionably  contains  small  quantities  of  ammonia,  partially  in  a 
state  of  carbonate  of  ammonia;  perhaps,  too,  partially  in  a  state  of  ni- 
trate, or  even  nitrite,  of  ammonia.  The  origin  of  this  substance  must 
evidently  be  attributed  principally  to  the  decomposition  of  vegetable 
and  animal  matter ;  and  its  presence  in  the  air  is  of  peculiar  importance 
in  regard  to  the  phenomena  of  vegetation  and  the  chemical  statics  of 
plants.  Several  chemists  have  attempted  to  determine  its  exact  propor- 
tion, which  does  not  seem  to  exceed  a  few  millionths  of  the  volume  of 
the  air. 

The  quantity  of  ammonia  found  in  different  waters  is  (in  weight) : 

Rain-water O'OOOOOOS 

Fresh-water 0'0000002 

Spring-water O'OOOOOOl 

From  one  to  two  grains  of  ammonia  per  cubic  foot  have  been  found 
in  sea- water.  This  is,  no  doubt,  a  very  trifling  quantity ;  but  when  we 
reflect  that  the  ocean  covers  more  than  three-quarters  of  the  globe,  and 
when  we  consider  also  its  enormous  mass,  it  may  be  fairly  looked  upon 
as  a  vast  reservoir  of  ammoniacal  salts,  whence  the  atmosphere  can 
make  good  the  losses  which  it  is  continually  undergoing. 

The  streams,  too,  carry  to  the  sea  prodigious  quantities  of  ammoniacal 
matter.  I  will  give  one  instance.  According  to  M.  Desfontaines,  the 
engineer,  the  Rhine  at  Lauterburg  has,  on  the  average,  a  flow  of  39,000 
cubic  feet  of  water  a  second;  and  from  a  careful  examination, of  the 
amount  of  ammonia  contained  in  the  water,  it  results  that  the  Rhine,  in 
its  passage  by  Lauterburg.  carries  down  with  it  every  twenty -four 
hours  at  least  22,500  Ibs.  of  ammonia— that  is,  13,000,000  Ibs.  a  year. 
The  atmosphere,  incessantly  undergoing  change  (although  its  constitu- 
tion remains  unaltered)  by  the  immense  labor  of  human  beings  who, 
like  so  many  chemical  pairs  of  bellows,  are  in  continual  motion  on  the 
bed  of  the  aerial  ocean,  is  the  theatre  of  accidental  chemical  modifica- 
tions which  play  their  part  in  the  general  organization.  We  see  rising 
from  the  ground  aqueous  vapor,  effluvia  of  carbonic  acid  gas,  nearly 
always  unmixed  with  nitrogen,  sulphureted  hydrogen  gas,  sulphurous 
vapors ;  less  frequently  we  notice  vapors  of  sulphuric  or  hydrochloric 
acid;  and,  lastly,  carbureted  hydrogen  gas,  which  has  for  thousands  of 


CHEMICAL  COMPONENTS  OF  THE  AIR.  67 

years  been  in  use  among  different  nations  for  the  purposes  of  producing 
warmth  and  light. 

Of  all  these  gaseous  emanations  the  most  numerous  and  abundant  are 
those  of  carbonic  acid.  In  former  ages,  the  greater  heat  of  the  globe 
and  the  large  number  of  crevices  that  the  igneous  rocks  had  not  yet 
covered  contributed  considerably  to  these  emissions.  Large  quantities 
of  hot  vapor  and  of  this  gas  became  mixed  with  the  aerial  fluid,  and 
produced  that  exuberant  vegetation  of  pit-coal  and  lignites  which  is 
nearly  an  inexhaustible  source  of  physical  strength  for  a  nation.  The 
enormous  quantity  of  carbonic  acid  the  combination  of  which  with  lime 
has  produced  the  chalky  rocks  then  rose  out  of  the  bosom  of  the  earth 
under  the  predominant  influence  of  volcanic  forces.  What  the  alkaline 
soils  could  not  absorb  spread  itself  into  the  air,  whence  the  vegetable 
matter  of  the  Old  World  drew  continuous  sustenance.  Then,  too,  abun- 
dant emissions  of  sulphuric  acid  in  vapor  have  led  to  the  destruction  of 
mollusks  and  fish,  and  to  the  formation  of  beds  of  gypsum.  Humboldt 
adds,  that  the  introduction  of  carbonate  of  ammonia  into  the  air  is  prob- 
ably anterior  to  the  appearance  of  organic  life  upon  the  globe's  surface. 
Besides  the  ammoniacal  vapors,  the  atmosphere  also  contains  many 
traces  of  nitrogen,  and  even  nitric  acid.  Several  observers  have  also 
demonstrated,  especially  in  large  towns,  the  presence  of  a  small  quanti- 
ty of  hydrogen  in  some  form,  probably  carbureted.  M.  Boussingault  was 
the  first  to  prove,  by  precise  experiments,  the  presence  of  a  hydrogenous 
gas  or  vapor  equal,  at  the  most,  to  a  10500  part  of  the  air  in  volume. 

Analysis  has  also  brought  to  light  a  certain  quantity  of  iodine.  The 
entire,  or  nearly  entire,  absence  of  iodine  in  the  air  or  water  of  certain 
mountainous  countries  has,  according  to  M.  Chatin,  a  close  connection 
with  the  existence  of  goitre  among  the  inhabitants  of  these  countries. 
His  conclusions  have  been  received,  as  a  rule,  with  incredulity  by 
chemists.  Yet,  when  we  consider  that  rain-water  collected  in  a  plu- 
viometer contains  various  kinds  of  salts,  which  arise  from  the  washing 
of  the  dust  suspended  in  the  atmosphere,  and  that  chemists  have  often 
found  evidence  of  the  presence  of  iodine  in  rain-water,  there  can  be  no 
difficulty  in  admitting  that  the  presence  in  the  air  of  iodine,  free  or  in 
combination,  may  be,  if  not  a  normal,  at  least  an  occasional  occurrence. 
We  now  arrive  at  the  last  element  ascertained  by  special  investigations 
to  be  existent  in  the  atmosphere,  viz.,  ozone. 

Van  Marum,  about  the  year  1780,  by  means  of  powerful  electric 
machines,  excited  a  large  number  of  sparks  in  a  tube  full  of  oxygen, 


gg  THE  ATMOSPHERE. 

about  six  or  seven  inches  long.  After  passing  about  five  hundred 
sparks  into  the  tube,  he  found  that  the  gas  had  acquired  a  very  strong 
smell,  which,  to  use  his  own  words,  "  seemed  clearly  the  smell  of  elec- 
tric matter."  Every  one,  indeed,  is  aware  that  if  lightning  strikes  any 
object  it  leaves  behind  it  what  is  commonly  called  a  sulphurous  smell. 
Van  Marum  also  found  that  the  gas  acquired,  after  the  experiment,  the 
property  of  oxidizing  mercury  without  heat.  Nearly  sixty  years  later, 
in  1839,  M.  Schoenbein,  professor  at  Basle,  informed  the  Academy  of 
Sciences  at  Munich  that,  having  decomposed  some  water,  he  had  been 
struck  by  the  smell  of  gas  emitted.  After  a  few  researches  he  drew 
the  conclusion  that  a  new  body  was  brought  to  light  by  his  experi- 
ment, which  he  called  ozone,  from  o%w  (to  emit  an  odor).  A  large 
number  of  contributions  have  been  subsequently  made  to  the  subject 
by  various  savants. 

Ozone  is  interesting  in  a  chemical  point  of  view,  both  in  its  nature 
and  its  energetic  affinities,  for  it  oxidizes  directly  both  silver  and  mer- 
cury, at  least  when  these  metals  are  moist.  It  also  liberates  iodine 
from  potassic  iodide,  and  forms,  with  the  metal,  an  oxide  which,  doubt- 
less, contains  far  more  oxygen  than  the  potash.  The  hydracids  impart 
to  it  their  hydrogen.  The  salts  of  magnesium  become  decomposed  by 
its  contact  with  the  formation  of  peroxide.  Chlorine,  bromine,  and 
iodine,  pass,  when  moist,  under  the  influence  of  ozone,  into  chloric, 
bromic,  and  iodic  acid. 

This  agent  has  an  exciting  effect  upon  the  lungs,  provokes  coughing 
and  suffocation,  and  presents  all  the  characteristics  of  a  poisonous  sub- 
stance. 

Notwithstanding  all  the  researches  that  have  been  made  in  reference 
to  ozone,  the  knowledge  of  it  is,  from  a  physical  and  chemical  point  of 
view,  very  imperfect ;  a  fact  easy  to  understand  when  I  state  that  it 
is  impossible,  even  with  the  most  perfect  methods,  to  transform  more 
than  T-^  of  a  mass  of  oxygen  into  pure  ozone.  This  maximum 
reached,  action  ceases.  How  can  it  be  easy  to  study  a  body  which  is 
spread  over  at  least  1800  times  its  own  volume  of  another  gas?* 

It  has  occurred  to  several  experimentalists,  such  as  Schoenbein, 
Berigny,  Pouriau,  Bceckel,  Houzeau,  and  Scoutetten,  to  join  to  the 
ordinary  meteorological  observations  ozonometrical  observations  also. 

*  [By  a  continuous  electrical  discharge,  maintained  for  many  hours,  Andrews  and  Tait 
were  enabled  to  transform  into  ozone  one-twelfth  of  the  volume  of  oxygen  operated  on  — 
Phil.  Trans.,  I860.— ED.] 


CHEMICAL  COMPONENTS  OF  THE  AIR.  69 

M.  Schoenbein,  in  his  experiments,  boiled  'one  part  of  potassic  iodide, 
ten  parts  of  starch,  and  two  hundred  of  water,  a  preparation  of  "Jo- 
seph's paper "  being  afterward  steeped  in  it.  The  latter  is  dried  in  a 
close  room,  and  then  cut  up  into  small  strips.  This  paper  becomes 
blue  by  contact  with  the  ozone,  for  the  iodine  is  set  at  liberty  and  re- 
acts upon  the  starch.  The  deepness  of  the  tint,  however,  depends  upon 
the  quantity  of  oxygen  which  has  been  turned  into  ozone.  A  small 
strip  is  exposed  each  day  for  twelve  hours,  sheltered  both  from  the 
sun's  rays  and  the  rain,  and  its  tint  is  then  compared  with  a  scale  of 
ten  colors,  varying  from  white  to  indigo. 

In  l^ol,  MM.  Marignac  and  De  la  Rive  undertook  several  experi- 
mental researches  as  to  ozone ;  and  their  conclusion  was,  that  this  sub- 
stance must  be  simply  oxygen  in  a  particular  condition  of  chemical  ac- 
tivity, determined  by  electricity.  Berzelius  and  Faraday  gave  their  ad- 
hesion to  this  opinion  of  the  Geneva  savants;  and  MM.  Fremy  and  Bec- 
querel  demonstrated,  by  fresh  experiments  in  1852,  its  legitimacy.  The 
works  of  Thomas  Andrews,  published  in  1855,  leave  no  doubt  upon 
this  head.  Ozone,  no  matter  from  what  source  it  is  derived,  is  a  unique 
and  separate  body,  with  identical  properties  and  the  same  constitution ; 
it  is  not  a  composite  body,  but  an  allotropic  condition  of  oxygen.  This 
allotropic  condition  is  due  to  the  action  of  electricity  upon  the  oxygen. 
This  opinion,  based  upon  the  best  experiments,  has  now  been  univer- 
sally accepted,  and  this  constitution  of  ozone  appears  incontestable. 

Let  us  further  add  to  all  these  divers  substances  the  presence  of  oxy- 
genated water,  as  indicated  by  M.  Struve,  director  of  the  Pulkowa  Ob- 
servatory. While  engaged  in  a  chemical  analysis  of  the  water  in  the 
River  Kusa,  M.  Struve  was  struck  with  the  presence  of  a  certain  quan- 
tity of  nitrite  of  ammonia,  which  was  only  to  be  found  after  a  fall  of 
snow  or  of  rain.  Soon  after  the  downfall  had  ceased,  all  trace  of  this 
substance  had  again  disappeared;  M.  Struve  therefore  supposed  that 
the  nitrite  of  ammonia  existed  in  the  air,  and  that  it  had  been  brought 
away  by  the  snow  or  the  rain.  He  entered  upon  researches  on  the  sub- 
ject, and  in  the  course  of  them  made  the  interesting  discovery  of  the 
presence  of  oxygenated  water  in  the  atmosphere.  From  these  research- 
es may  be  drawn  the  following  conclusions  :  1st.  Oxygenated  water  is 
formed  in  the  atmosphere  like  ozone  and  nitrite  of  ammonia,  and  be- 
comes separated  from  the  air  through  the  atmospheric  deposits.  2d. 
Ozone,  oxygenated  water,  and  nitrite  of  ammonia,  are  always  intimately 
connected.  3d.  The  alterations  which  the  atmospheric  air  brings  about 


-Q  THE  ATMOSPHERE, 

in  the  starch-iodine  papers  are  caused  by  the  ozone  and  oxygenated 

WaOne  word  more.  In  absorbing  into  our  lungs  the  quantity  of  air  due 
to  us  we  often  unwittingly  inhale  whole  hosts  of  microscopical  animals 
which  are  in  suspension  in  the  atmospheric  fluid,  and  even  portions  of 
antediluvian  animals,  mummies,  and  skeletons  of  past  ages! 

Paris  is  nearly  entirely  built  with  chalky  microscopical  skeletons  and 
tortoise-shells.  The  shells  of  the  /oramm/era,  for  instance,  m  a  fossil 
state  by  themselves  form  entire  chains  of  lofty  hills  and  immense  beds 
of  building-stone.  The  rough  chalk  in  the  neighborhood  of  Paris  is  in 
some  places  so  full  of  these  remains  that  a  cubic  inch  in  the  Gentilly 
quarries  contains  at  least  100,000  of  them.  When  we  pass  close  by  a 
house  that  is  being  pulled  down,  or  one  in  course  of  construction,  and 
find  ourselves  enveloped  in  a  cloud  of  dust  that  penetrates  down  our 
throats,  we  often,  beyond  a  doubt,  inhale  hundreds  of  these  tiny  atoms. 
Each  day  and  each  hour  we  inhale  and  take  into  our  chest  legions  of 
animal  and  vegetable  life.  There  are  the  living  microzoa,  several  spe- 
cies of  which  are  the  fish  of  our 'blood ;  there  are  the  vibriones,  which 
attach  themselves  to  our  teeth  like  oyster-banks  to  rocks.  Then,  again, 
there  is  the  dust  of  microscopical .  animalcules,  so  small  that  it  takes 
75,000,000  to  make  a  grain ;  and,  besides  these,  there  are  the  grains  of 
pollen  which,  germinating  in  our  lungs,  further  the  spread  of  parasite 
life,  which  is  out  of  all  comparison  more  developed  than  the  normal  life 
visible  to  our  eyes. 

The  winds  and  storms,  by  their  violent  agitation  of  the  atmosphere ; 
the  ascending  currents  due  to  the  inequalities  of  temperature ;  the  vol- 
canoes, by  their  incessant  emission  of  gas,  vapors,  and  ashes,  so  finely 
divided  that  they  often  fall  at  a  prodigious  distance,  carry  up  and  main- 
tain in  the  higher  regions  corpuscles  drawn  away  from  the  surface  of 
the  ground,  or  forced  out  of  the  internal  and,  perhaps,  still  incandescent 
portion  of  the  globe.  In  the  phenomena  connected  with  the  organism 
of  plants  and  animals,  these  substances,  so  slight  and  of  such  different 
origins,  the  vehicle  of  communication  for  which  is  the  air,  very  proba- 
bly exercise  a  far  more  pronounced  action  than  is  generally  believed. 
Their  permanence  is,  too,  placed  beyond  doubt  by  the  mere  evidence 
of  the  senses,  when  a  ray  of  sun  penetrates  a  darkened  room.  As  M. 
Boussingault  remarks,  "The  imagination  may  conceive  very  readily, 
though  not  without  a  certain  disgust,  what  is  contained  in  these  morsels 
of  dust  which  we  are  incessantly  inhaling,  and  which  have  been  aptly 


CHEMICAL  COMPONENTS  OF  THE  AIR.  71 

denominated  the  refuse  of  the  atmosphere.  They  establish,  in  a  certain 
sense,  a  contact  between  individuals  far  removed  from  each  other ;  and 
though  their  proportion,  their  nature,  and,  consequently,  their  effects, 
are  so  varied,  it  is  not  too  much  to  attribute  to  them  a  part  of  the  insa- 
lubrity which  generally  manifests  itself  in  all  great  agglomerations  of 
human  beings." 

Eain  carries  away  these  morsels  of  dust,  while  it  dissolves  their  solu- 
ble matter,  among  which  are  found  ammoniacal  salts,  as  they  also  dis- 
solve the  vapor  of  carbonate  of  ammonia  and  the  carbonic  acid  gas 
diffused  in  the  air.  There  must,  therefore,  exist  in  a  fall  of  rain,  at  its 
commencement,  more  soluble  substances  than  at  its  close ;  and  if  the 
rain  continues  uninterruptedly  in  calm  weather,  after  a  certain  interval 
there  can  only  be  very  insignificant  indications  of  the  existence  of  the 
substances. 

Miasmas,  the  propagators  of  epidemics,  are  superinduced  by  the 
aerial  currents ;  the  cholera,  the  small-pox,  the  yellow  fever,  and  the 
diseases  which  periodically  attack  a  district,  seem  to  have  their  princi- 
pal source  of  propagation  in  the  atmosphere — the  factory  of  death  as  it 
is  of  life.  The  rate  of  mortality,  which  was  so  heavy  in  Paris  during 
the  early  part  of  1870,  in  consequence  of  small-pox,  pleurisy,  and  in- 
flammation of  the  lungs,  was  especially  severe  in  the  northern  districts 
of  the  city,  over  which  the  southerly  wind  spread  the  miasmas  of  the 
whole  town,  and  where  there  was  scarcely  any  ozone.  A  knowledge 
of  the  conditions  of  public  health  will  be  furnished  in  part  by  a  study 
of  the  relations  of  meteorology  to  the  variations  in  the  rate  of  mortal- 
ity, which  is  as  continually  oscillating  under  the  slight  breath  of  the 
wind  as  under  the  trifling  alterations  in  barometrical  pressure. 

The  air  which  Gay-Lussac  brought  down  with  him  from  his  aero- 
nautical voyage,  and  which  was  collected  at  a  height  of  23,000  feet, 
had  the  same  composition  as  that  which  floats  upon  the  earth's  sur- 
face. The  experiments  of  M.  Boussingault  in  America,  and  those  of 
M.  Brunner  in  the  Alps,  lead  to  the  same  conclusions.  This  similarity 
in  results  arises  from  the  fact  that  currents  of  air  and  continual  varia- 
tions in  density  are  unceasingly  mixing  up  together  the  atmospheric 
strata. 

Is  it  the  same  at  a  greater  height  ?  It  is  scarcely  probable,  for  the 
nitrogen  and  oxygen  being  in  a  state  of  mixture,  and  not  chemically 
combined,  the  gases  must  be  ranged  according  to  their  density,  allow- 
ing, of  course,  for  the  law  of  expansion ;  that  is  to  say,  there  are,  as  it 


72  THE  ATMOSPHERE. 

were  two  distinct  atmospheres,  the  least  dense  of  which  does  not  ex- 
tend'so  far  as  the  other,  so  that  the  proportion  of  nitrogen,  the  density 
of  which  is  0-972,  that  of  the  air  being  1,  must  increase  the  higher  one 
rises  in  the  atmosphere;  while  the  oxygen,  the  density  of  which  is 
1-057  (and  which  is  the  denser  of  the  two),  must  be  in  a  greater  pro- 
portion near  the  surface.  According  to  this  hypothesis,  the  latter  gas, 
at  23,000  feet,  would  constitute  only  Vw  of  the  volume  of  air  ;  but  at 
present  experiment  has  failed  to  note  so  great  a  difference,  because 
this  calculation  supposes  the  air  to  be  in  a  state  of  tranquillity,  whereas 
at  these  heights  it  is,  as  a  matter  of  fact,  in  a  continuous  state  of  agi- 
tation. 

The  composition  of  the  air  varies  very  little :  when  it  rains,  the  con- 
densed water  dissolves  more  oxygen  than  nitrogen ;  in  frost,  the  water 
leaves  these  two  gases  alone ;  the  water  which  evaporates  returns  then 
to  the  atmosphere. 

We  may  now  ask  ourselves,  in  terminating  this  study  of  the  chem- 
ical composition  of  the  air,  if  this  constitution  is  variable  over  the  ter- 
restrial globe.  By  virtue  of  one  of  the  great  natural  harmonies  which 
unite  the  animal  and  the  vegetable  kingdoms,  while  the  animals  act  as 
combustion-machines,  taking  the  oxygen  from  the  air  and  throwing  it 
back  into  the  atmosphere  in  the  state  of  carbonic  acid,  the  vegetables 
play  the  reverse  part,  acting  as  reducing-machines.  Under  the  influ- 
ence of  the  solar  rays,  the  green  portions  of  the  plants  react  upon  the 
carbonic  acid,  decompose  it,  concentrate  the  carbon,  and  restore  the 
oxygen  to  the  air.  The  atmosphere,  vitiated  by  the  animals,  is  puri- 
fied by  the  action  of  the  vegetables.  The  chemical  equilibrium  of  the 
air's  components  has  thus  a  tendency  to  self-preservation  by  virtue  of 
this  inverse  action  brought  to  bear  upon  its  constituent  elements. 

Certain  phenomena  due  to  the  decomposition  of  rocks  through  oxida- 
tion seemed,  at  first  sight,  calculated  to  modify  in  the  long  run  the  com- 
position of  the  air ;  but  a  series  of  inverse  actions  of  reduction  tends  to 
restore,  in  the  shape  of  carbonic  acid,  the  oxygen  that  has  disappeared. 
As  Ebelmen  has  pointed  out,' in  his  memoir  upon  changes  in  rocks,  the 
process  of  reactions  in  the  mineral  matter  upon  the  globe's  surface  seems 
also  calculated  to  establish  a  compensation  which  maintains  the  chem- 
ical composition  of  the  atmosphere. 

The  question  is  whether  this  compensation  is  complete.  Supposing 
it  does  not  take  place — as,  indeed,  is  possible — does  the  quantity  of  oxy- 
gen diminish?  As  Thenard  has  remarked,  "This  is  a  very  important 


CHEMICAL  COMPONENTS  OF  THE  AIR.  73 

question,  the  solution  of  which  can  only  be  arrived  at  in  the  course  of 
several  centuries,  because  of  the  enormous  volume  of  air  by  which  our 
planet  is  surrounded." 

In  their  remarkable  memoir  upon  the  true  constitution  of  the  atmos- 
pheric air,  MM.  Dumas  and  Boussingault  thus  expressed  themselves  in 
1841: 

"Some  calculations,  which,  though  not  of  absolute  precision,  never- 
theless are  based  upon  sufficiently  certain  grounds,  tend  to  prove  how 
far  an  analysis  should  extend  to  reach  the  limit  at  which  the  varia- 
tions 'in  oxygen  would  be  sensibly  manifest.  The  atmosphere  is  un- 
ceasingly agitated;  the  currents,  stirred  up  by  heat,  by  winds,  by  elec- 
tric phenomena,  are  continually  being  mixed  up  and  confusing  to- 
gether the  various  strata.  The  whole  mass  would,  therefore,  have  to 
be  changed  in  order  to  admit  of  an  analysis  indicating  the  difference 
between  one  epoch  and  another.  But  this  mass  is'  enormous.  If  we 
could  place  the  whole  atmosphere  into  a  balloon,  and  suspend  it  in  one 
side  of  a  pair  of  scales,  it  would  be  necessary  to  put  on  the  other  side 
138,000  cubes  of  copper  (each  a  mile  in  length,  breadth,  and  thickness) 
to  balance  it.  Let  us  now  suppose  that  each  man  consumes  a  little  more 
than  two  pounds  of  oxygen  a  day,  that  there  are  a  thousand  millions 
of  men  upon  the  earth,  and  that,  through  the  respiration  of  animals  and 
the  putrefaction  of  organic  matter,  this  consumption  attributed  to  man 
be  quadrupled.  Let  us  further  suppose  that  the  oxygen  disengaged 
from  plants  is  only  the  compensating  agent  of  the  causes  of  absorp- 
tion omitted  in  our  calculation,  which  would  assuredly  be  putting  the 
chances  of  alteration  of  the  air  in  the  strongest  light.  Well,  even 'on 
this  overdrawn  hypothesis,  at  the  end  of  a  century  the  whole  human 
race,  and  three  times  its  equivalent,  would  only  have  absorbed  a  quan- 
tity of  oxygen  equal  to  fourteen  or  fifteen  of  the  cubic  miles  of  copper. 

"Thus,  to  assert  that,  with  their  utmost  efforts,  the  animals  which 
people  the  face  of  the  earth  could  in  a  century  render  the  air  they 
breathe  impure,  to  the  extent  of  depriving  it  of  the  -^^  part  of  the 
oxygen  that  nature  has  placed  there,  is  to  make  a  supposition  far  be- 
yond the  reality." 

In  habitations  badly  ventilated,  the  effects  of  the  breathing  of  men 
or  animals,  and  the  phenomena  of  the  combustion  of  coal  or  of  com- 
bustible matters,  may  cause  a  sensible  alteration  in  the  state  of  the  air. 
Thus,  in  barracks,  hospital  rooms,  theatres,  wells,  mines,  etc.,  chemical 
analysis,  when  it  is  accurate  enough,  indicates  a  different  composition 


74  THE  ATMOSPHERE. 

from  that  of  the  open  air.  Furthermore,  in  habitations  even  out  of  the 
influence  of  the  presence  of  sick  persons,  the  animal  emanations  which 
escape  with  the  aqueous  vapor  in  respiration  and  perspiration  may  ex- 
ercise an  incontestable  physiological  influence,  often  more  injurious 
than  that  caused  by  the  production  of  carbonic  acid  or  the  disappear- 
ance of  the  oxygen  in  small  quantities. 

It  is  especially  when  the  air  arrives  at  a  state  of  saturation  from  the 
causes  cited  above  that  there  is  reason  to  consider  it  deleterious.  There 
is  an  unanimity  of  opinion  in  the  present  day  that,  to  avoid  a  disas- 
trous influence  upon  the  organic  economy,  dwelling-houses,  and  espe- 
cially hospitals,  should  be  so  constructed  as  to  give  more  than  20,000 
cubic  feet  of  air  per  day  to  each  individual. 


SOUND  AND  THE  VOICE. 


75 


CHAPTER  VI. 

SOUND  AND  THE  VOICE. 

AMONG  the  works  of  the  atmosphere  in  terrestrial  life,  one  of  the 
most  important  is  unquestionably  that  of  serving  as  a  vehicle  for  hu- 
man thought,  and  enveloping  the  world  in  a  sphere  of  harmony  and 
activity  which  could  not  exist  without  it. 
What  is  sound? 

It  is  a  movement  produced  in  the  air,  and  transmitted  therein  by 
successive  undulations.     To  be  perceived  by  the  ear,  this  vibratory 
movement  must  be  neither  too  slow  nor  too  rapid.     When  the  air, 
agitated  by  sound,  vibrates  at  the  rate  of 
sixty  undulations   a  second,  it  emits  the 
dullest  sound   which   can    reach   the   ear. 
When  the  vibrations  are  40,000  per  second, 
they  convey  the  sharpest  sound  which  the 
auditory  nerves  can  perceive. 

To  appreciate  the  nature  of  the  sonorous 
movement,  let  us  suppose  that  between  the 
chaps  in  a  vise,  A  (see  Fig.  22),  is  fixed  one 
of  the  extremities,  c,  of  an  elastic  blade, 
c  D ;  that  the  upper  end,  D,  is  pulled  back 
to  D',  and  then  let  go.  By  virtue  of  its 
elasticity,  the  blade  will  return  to  its  primi- 
tive position;  but  in  consequence  of  the 
speed  it  has  acquired,  it  will  pass  it  and  go 
on  to  D",  executing  on  both  sides  of  c  D  a 
series  of  oscillations  the  amplitude  of  which 
will  gradually  decrease,  and  in  a  more  or 

Fig.  22,-Vibrations  of  a  blade.         ^^  ^^  space  Qf  ^  altogether  cease< 

The  longer  the  elastic  blade  is,  the  slower  will  be  the  vibrations; 
while,  in  proportion  as  the  blade  is  shortened,  the  vibratory  movement 
will  become  more  rapid,  and  at  a  certain  point  will  be  imperceptible  to 
the  eye.  But  when  the  organ  of  vision  ceases  to  play  a  part,  so  to 
speak,  that  of  the  organ  of  hearing  begins,  and  the  ear  can  distinctly 


,-Q  THE  ATMOSPHERE. 

catch  a  sound,  the  nature  of  which  depends  upon  the  physical  condi- 
tions of  the  vibrating  body.  Another  instance  of  the  production  of 
sound  is  furnished  by  the  vibration  of  a  piece  of  cord  fas- 
tened at  its  extremities,  A  B,  and  pulled  in  the  middle.* 
Its  vibration  is  rendered  perceptible  by  the  fact  of  the 
cord  presenting  the  shape  of  a  bobbin.  By  reason  of  the 
persistent  impression  upon  the  retina,  and  the  speed  of 
the  vibratory  movement,  the  eye  sees  the  cord  in  all  its 
positions  together,  as  it  were,  the  time  of  a  vibration 
being  less  than  that  of  a  luminous  impression,  which  is 
the  tenth  of  a  second.  Sound,  therefore,  is  but  an  im- 
pression upon  the  organ  of  hearing,  caused  by  the  vibra- 
ting movement  of  a  given  body.  But  the  existence  of  a 
vibratory  body  on  the  one  hand,  and  of  an  ear  on  the 
other,  is  not  enough  to  cause  an  impression:  a  relation 
must  be  established  between  that  body  and  the  organ  of 
hearing,  and  this  is  effected  by  a  ponderable  medium, 
liquid  or  gaseous,  constituted  of  more  or  less  elastic  matter.  If  we  im- 
agine a  body  vibrating  in  a  complete  vacuum,  or  in  the  centre  of  a 
space  entirely  devoid  of  elasticity,  the  ear,  at  a  certain  distance  off, 
would  catch  no  sound.  Sound,  in  the  proper  sense  of  the  word,  does 
not  exist  in  such  a  case. 

We  may  in  fact  form,  from  what  is  mentioned  above,  the  following- 
definition  of  sound: 

Sound  is  an  impression  produced  by  the  vibrations  of  a  body  transmitted 
to  the  organ  of  hearing  by  the  intervention  of  a  ponderabk  and  elastic  me- 
dium. 

At  what  rate  is  sound  propagated  ? 

The  first  exact  measurements  were  made  in  1738  by  a  commission 
of  the  Academy  of  Sciences,  of  which  Lacaille  and  Cassini  de  Thury 
were  members.  Several  pieces  of  ordnance  were  placed  upon  the 
heights  of  Montmartre  (then  outside  the  walls  of  Paris)  and  at  Mon- 
tlhery  (an  elevated  position  in  the  department  of  the  Seine-et-Oise,  dis- 
tant about  16  miles  from  Paris),  and  it  was  arranged  that  from  a  given 
hour  a  gun  should  be  fired  at  equal  stated  intervals.  The  persons  en- 
gaged in  the  experiment  counted  the  .time  that  elapsed  between  the 
flash  and  the  arrival  of  the  report;  and  this  was. found  to  be,  on  an 

*  [This  is,  of  course,  the  principle  of  all  stringed  instruments — the  harp,  violin,  etc.  It  is 
difficult  to  hold  the  cord  sufficiently  tight  by  the  hand  to  produce  a  note.— ED.] 


SOUND  AND   THE  VOICE. 


77 


average,  1  minute  24  seconds  for  a  distance  of  about  95,000  feet,  which 
is  at  the  rate  of  1037  feet  per  second. 

These  experiments  were  repeated  in  1822  by  the  Bureau  des  Longi- 
tudes— a  section  of  the  Academy  of  Sciences — the  persons  taking  part 
in  them  being  Arago,  Gay-Lussac,  Humboldt,  Prony,  Bouvard,  and 
Mathieu.  Villejuif  and  Montlhery,  distant  from  each  other  61,000 
feet,  were  the  places  selected ;  and  it  was  found  that  at  a  temperature 
of  61°  the  velocity  of  transmission  was  1047  feet  a  second. 

A  great  number  of  similar  experiments  have  been  made  in  different 
countries.  Yery  recently,  M.  Regnault  investigated  this  subject,  em- 
ploying all  the  resources  of  modern  physics,  and  especially  telegraphic 
signals,  for  registering  instantaneously  the  discharge  and  the  arrival  of 
the  sound. 

The  velocity  of  sound  varies  with  the  density  and  the  elasticity  of 
the  air,  and  therefore  with  its  temperature.  According  to  the  most 
accurate  measurements,  the  following  table  may  be  given  in  reference 
thereto : 


Temperature 
(Fahr.) 

Velocity 
per  Second. 

Temperature 
(Fahr.) 

Velocity 
per  Second. 

5 

14° 
23°   . 
32° 
41° 
50° 
68° 

1056  feet. 
1070 
1079 
1089 
1096 
1102 
1112 

68° 
77° 
80° 
95° 
104° 
113° 
122° 

1122  feet. 
1132 
1142 
1152 
1161 
1171 
1181 

Sound  is  propagated  in  the  air  by  successive  undulations,  which  may 
be  roughly  compared  to  the  circular  waves  which  are  produced  on  the 
surface  of  water  around  a  point  disturbed  by  the  fall  of  a  stone.  But 
they  are,  in  reality,  very  different  phenomena.  In  the  liquid  waves, 
the  molecules  are  alternately  raised  and  lowered  in  regard  to  the  gen- 
eral level,  but  undergo  no  change  of  density ;""  while  this  change  is,  on 
the  contrary,  a  characteristic  of  the  waves  of  sound.  There  is,  however, 
one  circumstance  common  to  both  these  phenomena  which  is  worth 
pointing  out — and  that  is,  that  the  wave  causes  no  real  progressive 
movement.  Thus,  when  waves  of  water  follow  each  other,  if  we  notice 
any  small  floating  object,  it  is  seen  to  alternately  rise  and  fall,  but  it  re- 
mains in  the  same  place  upon  the  surface  of  the  water.  Similarly,  in 
the  waves  of  sound,  the  molecules  of  the  air  execute  oscillatory  move- 
ments in  regard  to  the  propagation  of  sound,  but  the  centre  of  these 
movements  remains  unchanged. 


irg  THE  ATMOSPHERE. 

Scientific  education  should  teach  us  to  behold  in  nature  the  invisible 
as  well  as  the  visible— to  depict  to  the  eyes  of  the  intellect  what  escapes 
the  eyes  of  the  body.  We  may,  with  a  little  application,  form  a  true 
idea  of  a  sound-wave ;  we  may  mentally  see  the  molecules  of  air  first 
pressed  the  one  against  the  other;  then,  immediately  after,  this  con- 
densation brought  away  again  by  an  opposite  effect  of  dilatation  or 
rarefaction.  We  thus  learn  that  a  wave  of  sound  is  composed  of  two 
parts :  in  one  the  air  is  condensed ;  while  in  the  other,  on  the  contrary, 
it  is  rarefied.  A  condensation  and  a  dilatation  are  then  the  essential 
constituents  of  a  sound-wave.  But,  if  the  air  is  necessary  to  the  propa- 
gation of  sound,  what  happens  when  a  sounding  body,  such  as  the  bell 
of  a  clock,  is  placed  in  a  space  destitute  of  air  ?  The  result  is  that  no 
sound  proceeds  from  the  empty  space ;  the  hammer  strikes  the  bell, 
but  silently.  Hawksbee  demonstrated  this  fact  in  a  memorable  experi- 
ment in  1705,  before  the  Eoyal  Society  of  London.  He  placed  a  clock 
under  the  receiver  of  an  air-pump,  in  such  a  way  that  the  striking  of 
the  clapper  would  continue  after  the  air  had  been  exhausted.  While 
the  receiver  was  full  of  air,  the  sound  was  quite  audible;  but  it  was  no 
longer  so  (or  at  least  in  a  very  slight  degree)  when  a  vacuum  had  been 
created.  The  appended  illustration  is  that  of  a  contrivance  which  en- 
ables us  to  repeat  Hawksbee's  experiment 
in  an  improved  manner.  Under  the  re- 
ceiver B,  placed  firmly  on  the  plate  of  an 
air-pump,  will  be  seen  the  works  of  a  strik- 
ing clock,  A.  The  hammer  is  kept  back  by 
a  spring  and  ratchet,  c.  As  much  as  possi- 
ble of  the  air  is  exhausted ;  then,  by  means 
of  a  stem,  ^,  which  passes  out  through  the 
top  of  the  receiver,  without  letting  in  the 
exterior  air,  the  trigger  d,  which  holds  back 
the  hammer  b,  is  pulled.  The  bell,  a,  vi- 
brates silently.  But  if  we  let  the  air  into 
the  recipient,  we  at  once  hear  a  sound,  very 
feeble  at  first,  but  growing  louder  as  the  air 
becomes  denser.  At  great  heights  in  the  at- 
Pig  ^  mosphere,  the  intensity  of  sound  is  notably 

less.    According  to  the  calculations  of  Saus- 

sure,  the  detonation  of  a  pistol  upon  the  summit  of  Mont  Blanc  is  about 
equal  in  intensity  to  that  of  a  common  cracker  at  the  level  of  the  sea 


SOUND  AND  THE  VOICE.  79 

Since  it  is  proved  that  there  is  no  sound  in  a  vacuum,  fearful  catas- 
trophes might  take  place  in  the  planetary  regions  without  the  slightest 
audible  notice  of  them  reaching  the  surface  of  the  earth. 

The  vibratory  movement  of  the  air  has  been  represented  as  being  a 
circular  wave,  which  spreads  out  in  all  directions  with  equal  velocity, 
and  diminishes  in  intensity  as  it  advances.  Where  does  it  cease  ? 
where  is  it  extinguished  ?  We  must  regard  this  as  taking  place  at  the 
point  in  space  where  it  is  no  longer  sensible  to  the  most  delicate  ear ; 
and  we  all  know  how  much  this  limit  varies  with  the  organization  and 
habits  of  different  individuals.  At  the  same  time,  there  can  be  no 
doubt  that  the  aerial  wave  continues  to  spread  out  after  the  most  prac- 
ticed ear  has  ceased  to  be  sensible  of  it.  In  the  places  where  there  is 
a  numerous  population,  the  incessant  noise  kept  up  in  the  air  by  so 
many  thousands  of  people  creates  a  characteristic  difference  between 
day  and  night ;  the  noises  become  confounded  together,  and  are  propa- 
gated in  a  confused  mass.  During  the  night  there  is  nothing  to  lessen 
the  intensity  of  sound,  and  the  ear  perceives  in  all  their  force  the  howl- 
ing of  the  tempest,  the  blast  of  the  winds,  the  roaring  of  the  waves, 
the  shrill  cry  of  the  bird  of  prey  or  the  wild  beast ;  and  it  is  then  that 
pusillanimous  fears  and  superstitious  terror  take  possession  of  the  timid. 
Traveling  in  a  balloon  over  the  plains  of  Charente,  the  stream  of  a 
river  seemed  to  make  as  much  noise  as  that  of  a  great  cascade,  and  the 
croaking  of  the  frogs  were  audible  at  the  height  of  3000  feet.  Above 
two  miles  all  noise  ceases.  I  never  encountered  a  silence  more  com- 
plete and  solemn  than  in  the  heights  of  the  atmosphere — in  those  chill- 
ing solitudes  to  which  no  terrestrial  sound  reaches. 

"Two  conditions  determine  essentially,"  says  Tyndall,  "the  velocity 
of  the  sound-wave,  viz.,  the  elasticity  and  density  of  the  medium  which 
it  passes  through."  The  elasticity  of  the  air  is  measured  by  the  press- 
ure which  it  supports,  and  to  which  it  forms  an  equilibrium.  We  have 
seen  that,  at  the  level  of  the  sea,  this  pressure  is  equal  to  that  of  a  col- 
umn of  quicksilver  29'92  inches  high.  Upon  the  summit  of  Mont 
Blanc  the  barometrical  column  scarcely  exceeds  half  this  height,  and, 
therefore,  at  the  highest  point  of  this  mountain,  the  elasticity  of  the  air 
is  only  half  what  it  is  upon  the  sea-coast. 

If  we  could  increase  the  elasticity  of  the  air  without  at  the  same  time 
augmenting  its  density,  we  should  increase  the  velocity  of  sound.  We 
should  also  effect  that  object  if  we  could  diminish  the  density  without 
making  any  change  in  the  elasticity.  The  air  heated  in  a  closed  vessel, 


gO  THE  ATMOSPHERE. 

in  which  it  can  not  become  dilated,  has  its  elasticity  increased  by  the 
warmth,  while  its  density  remains  the  same.  Sound  will,  therefore,  be 
propagated  more  rapidly  through  air  thus  heated  than  through  the  air 
at  its  normal  temperature.  In  like  manner,  air  which  is  free  to  dilate 
has  its  density  diminished  by  heat,*  while  its  elasticity  remains  the 
same,  and  consequently  it  will  propagate  sound  more  rapidly  than  cold 
air— this,  indeed,  takes  place  when  our  atmosphere  is  heated  by  the 
sun ;  the  air  becomes  dilated  and  much  lighter,  volume  for  volume, 
while  its  pressure,  or,  in  other  words,  its  elasticity,  remains  the  same. 
This  is  the  explanation  of  the  statement  that  the  velocity  of  sound  in 
air  is  1090  feet  a  second  at  the  temperature  of  melting  ice.  At  a  lower 
temperature  the  velocity  is  less,  and  at  higher  temperatures  greater, 
with  an  average  difference  of  about  one  foot  for  one  degree  (Fahr.). 
Under  the  same  pressure — that  is  to  say,  with  the  same  elasticity — the 
density  of  hydrogen  is  much  less  than  that  of  the  air,  and,  in  conse- 
quence, the  velocity  of  sound  through  hydrogen  gas  considerably  ex- 
ceeds its  velocity  through  air.  The  reverse  is  the  case  with  carbonic 
gas,  which  is  denser  than  air ;  for  under  the  same  pressure  sound  trav- 
els less  rapidly  through  this  gas  than  through  air. 

The  fact  that  air,  even  when  very  rarefied,  can  transmit  intense 
sounds,  is  proved  by  the  explosion  of  meteors  at  a  great  height  above 
the  earth,  though  it  is  true  that,  for  this  to  be  the  case,  the  initial  cause 
of  the  atmospheric  disturbance  must  be  very  violent. 

The  movement  of  sound,  like  all  others,  is  less  in  amount  when  it 
communicates  from  a  light  body  to  one  more  dense.  The  action  of 
hydrogen  on  the  voice  is  a  phenomenon  of  this  kind.  The  voice  is 
formed  by  the  injection  of  air  from  the  lungs  into  the  larynx ;  in  its 
passage  through  this  organ  the  air  is  set  vibrating  by  the  vocal  chords, 
which  thus  give  rise  to  sound ;  and  if  one  wishes  to  speak  when  the 
lungs  are  full  of  hydrogen,  the  vocal  chords  still  impress  their  move- 
ment on  the  hydrogen,  which  transmits  it  to  the  air  outside.  But  this 
transmission  of  a  light  gas  to  one  much  denser  causes  a  considerable 
diminution  in  the  intensity  of  the  sound.  The  effect  of  this  is  very  re- 
markable. Tyndall  demonstrated  it  to  the  Royal  Institution  in  Lon- 
don. Having,  by  a  great  effort  of  inhalation,  filled  his  lungs  with 
hydrogen,  he  began  to  speak,  and  his  voice,  generally  powerful,  was 
hoarse  and  hollow ;  there  was  no  ring  in  it ;  it  seemed  to  issue  from 
the  depths  of  the  grave. 

*  [The  air  must  be  contained  in  a  vessel  so  constructed  that  the  elasticity  (pressure)  is  kept 
the  same  (for  instance,  in  a  cylinder  in  which  fits  a  piston  of  constant  weight).— ED.] 


SOUND  AND   THE  VOICE.  gl 

The  intensity  of  sound  mainly  depends  upon  the  density  of  the  air 
from  which  it  proceeds,  not  on  that  of  the  air  in  which  it  is  heard. 

The  wave  of  sound,  propagated  in  all  directions  from  the  point  where 
the  sound  has  been  produced,  diffuses  itself  in  the  mass  of  air  in  which 
the  motion  takes  place,  and  consequently  lessens  the  amount  of  move- 
ment at  any  point.  Let  us  imagine  around  the  centre  of  disturbance  a 
spherical  layer  of  air,  with  a  radius  of  a  yard ;  another  layer  of  the 
same  thickness,  with  a  radius  of  two  yards,  contains  four  times  as  much 
air;  one  with  a  radius  of  three  yards  contains  nine  times  as  much;  one 
with  a  radius  of  four  yards,  sixteen  times  as  much ;  and  so  on.  The 
quantity  of  matter  set  in  motion  increases,  therefore,  as  the  square  of 
the  distance  from  the  centre  of  disturbance ;  the  intensity  of  the  sound 
diminishes  in  the  same  degree.  This  law  is  expressed  by  the  statement 
that  the  intensity  of  sound  varies  inversely  as  the  square  of  the  dis- 
tance from  the  point  of  initial  disturbance.  The  decrease  in  the  sound 
in  inverse  ratio  to  the  square  of  the  distance  would  not  occur  if  the- 
sound-wave  spread  in  such  a  way  as  to  prevent  its  being  diffused  later- 
ally. By  producing  a  sound  in  a  tube  the  interior  surface  of  which  is 
perfectly  smooth  these  conditions  may  be  realized,  and  the  wave  thus 
confined  reaches  a  great  distance,  with  but  a  slight  loss  of  intensity. 
In  this  way  Biot,  noting  the  transmission  of  sound  through  the  conduit 
pipes  that  supply  Paris  with  water,  found  that  he  could  carry  on  a  con- 
versation in  a  low  tone  at  a  distance  of  3300  feet ;  the  faintest  murmur 
of  the  voice  was  heard  at  this  distance,  and  the  firing  of  a  pistol  at  one 
end  of  the  pipes  extinguished  a  candle  placed  at  the  other  end. 

Echoes  depend,  in  a  great  measure,  upon  the  compressibility  and 
elasticity  of  the  air.  The  sound-wave,  as  has  been  stated,  spreads  in- 
definitely, and  is  finally  lost  in  space;  but  if  it  encounters  a  body  capa- 
ble of  opposing  it,  it  undergoes  a  reflection  like  that  of  light  when  it 
falls  upon  a  smooth  surface.  For  an  echo  to  be  distinctly  produced, 
there  must  be  a  distance  of  fifty -five  feet  at  least — the  tenth  of  a  sec- 
ond in  time — between  the  person  speaking  and  the  reflecting  surface. 
When  the  former  is  nearer,  the  echo  is  replaced  by  a  confused  reso- 
nance, which,  in  some  buildings,  renders  it  impossible  for  a  speaker  to 
make  himself  heard. 

Whether  acute  or  grave,  sounds  have  the  same  velocity* — that  of 

*  [This  is  proved  by  the  fact  that,  if  a  band  of  music  be  heard  at  a  distance,  the  sounds  are 
not  confused,  the  distinctness  of  the  tune  being  unaffected  by  the  distance,  though  the  loudness 
is  of  course  diminished. — ED.] 

G 


go  THE  ATMOSPHERE. 

1115  feet  a  second  in  air  of  61°  (Fahr.).  At  half  this  distance  the  echo 
aives  back  four  syllables  rapidly  pronounced;  at  a  greater  distance  it 
will  distinctly  reflect  a  larger  number  of  syllables  and  whole  phrases. 
The  echo  in  Woodstock  Park  repeats  seventeen  syllables  in  the  day- 
time and  sixty  at  night.  Pliny  tells  us  that  a  portico  was  built  at 
Olympia  which  repeated  sounds  twenty  times.  The  echo  at  the  Cha- 
teau de  Simonetti  was  said  to  repeat  the  same  word  forty  times.  The 
theory  is  the  same  for  the  multiplied  echoes;  they  result  from  the  re- 
flecting surfaces  against  which  the  aerial  wave  is  thrown  back  several 
times  from  the  one  to  the  other,  like  a  ray  of  light  between  two  parallel 
glass  plates.  Perceptible  sounds  are  included  between  the  limits  of 
60,000  and  40,000  simple  vibrations  a  second,  except  in  the  case  of 
ears  which  are  exceptionally  sharp.  The  undulations  of  the  ether, 
which  produce  light,  are  far  more  rapid.*  Visible  colors  are  the  result 
of  vibrations  so  rapid  that  between  400  and  800  billions  take  place  in 
•  a  second. 

Of  perceptible  sounds,  the  extreme  limits  of  the  human  voice  are  the 
lowest,  /a,  of  87,  and  the  highest,  ut,  of  4200  vibrations. 

Sound  has  four  fundamental  properties— duration,  height,  intensity, 
and  timbre  or  quality.  The  first  three  are  defined  by  the  words  used 
to  express  them.  As  to  the  timbre,  it  is  the  resonance  peculiar  to  each 
instrument  and  to  each  voice  which  enables  us  to  clearly  distinguish 
the  sounds  of  a  violin  from  those  of  a  clarionet  or  a  flute,  and  to  recog- 
nize a  person  by  hearing  him  speak  or  sing.f 

The  timbre  of  sounds  has  long  been  an  insoluble  enigma  to  natural 
philosophers  and  physiologists.  It  is  only  within  the  last  few  years 
that  the  excellent  experiments  of  Helmholtz  have  proved  that  it  de- 
pends upon  the  number  of  harmonic  sounds  which  are  produced  simul- 
taneously with  the  fundamental  tone,  and  upon  their  relative  intensity. 
The  intensity  of  sounds  generated  upon  the  surface  of  the  earth 
spreads  upward  far  more  readily  than  in  any  other  direction,  and  is 
transmitted  to  great  heights  in  the  atmosphere.  Citing  some  few  in- 
stances from  my  aeronautical  travels,  I  will,  in  the  first  place,  mention 

*  [It  must  be  borne  in  mind  that  there  is  only  a  general  analogy  between  light  and  sound. 
In  the  latter,  the  vibrations  consist  of  condensations  and  rarefactions  in  the  air  (or  other  gas). 
which  are  longitudinal — i.  e.,  take  place  in  the  direction  in  which  the  sound  is  proceeding: 
while,  in  light,  the  vibrations  are  transversal  (i.  e.,  perpendicular  to  the  direction  of  the  ray), 
and  take  place  in  an  ether  which  is  supposed  to  pervade  all  space. — ED.] 

t  It  is,  of  course,  more  difficult  to  recognize  a  person  by  his  song  than  by  his  speech. 


SOUND  AND  THE  VOICE.  83, 

that  a  noise,  immense,  colossal,  and  indescribable,  is  ever  to  be  heard 
at  1000  to  1500  feet  above  Paris.  In  rising  from  a  relatively  quiet 
garden — as  from  the  Observatory — we  are  astonished  to  hear  a  chaos 
of  sound  and  a  thousand  various  noises.  The  following  details  will, 
however,  illustrate  more  strikingly  this  ascent  of  sound : 

The  whistle  of  a  steam-engine  may  be  heard  at  10,000  feet ;  the  noise 
of  a  train  at  8200  ;*  the  barking  of  a  dog  at  6000 ;  the  report  of  a  gun 
attains  the  same  height ;  the  shouts  of  people  sometimes  are  audible  at 
5000  feet,  as  also  the  crowing  of  a  cock  or  the  tolling  of  a  bell.  At 
4500  feet  the  beating  of  a  drum  and  the  sound  of  a  band  are  audible ; 
at  3900  feet  the  rumble  of  vehicles  upon  the  pavement ;  and  at  3300 
feet  the  shout  of  a  single  individual.  At  this  last  height,  during  the 
silence  of  the  night,  the  current  of  a  stream  at  all  rapid  produces  the 
same  effect  as  the  rush  of  a  cascade ;  and  at  2950  feet  the  croaking  of 
frogs  is  plaintively  distinct.  At  2620  feet  the  slight  noises  made  by 
the  cricket  are  heard  very  plainly. 

This  does  not  hold  good  of  sound  when  descending.  While  we  hear 
distinctly  the  voice  of  a  person  speaking  from  1600  feet  underneath  us, 
it  is  impossible  to  catch  what  is  said  at  a  height  of  more  than  300  feet 
above  us. 

The  occasion  upon  which  I  was  most  struck  by  this  astonishing 
transmission  of  sounds  vertically  upward  was  in  an  ascent  that  took 
place  on  June  23, 1867.  Having  been  in  the  midst  of  the  clouds  for 
several  minutes,  we  were  surrounded  by  a  white  and  opaque  veil  that 
concealed  both  the  sky  and  the  earth,  when  I  noticed  with  surprise  a 
singular  increase  of  light  taking  place  around  us,  and  all  at  once  the 
sounds  of  a  band  reached  our  ears.  We  could  follow  the  piece  of  music 
as  distinctly  as  if  the  band  had  been  in  the  clouds,  a  few  yards  distant 
from  us.  We  were  then  just  above  Antony,  a  village  near  Paris.  Hav- 
ing mentioned  the  fact  in  a  newspaper,  I  was  glad  to  receive,  a  few 
days  afterward,  a  letter  from  the  President  of  the  Philharmonic  Society 
in  that  place,  informing  me  that  his  society  had  seen  the  balloon  above 
them,  and  had  purposely  played  a  very  soft  piece,  in  the  hope  that 
they  might  be  of  service  to  us  in  our  researches. 

In  this  case  the  balloon  was  about  2950  feet  above  the  place.  At 
3280,  3940,  and  even  4590  feet,  the  parts  were  still  distinctly  audible. 
Far  from  being  an  obstacle  to  the  transmission  of  sound,  the  clouds  in- 
creased its  intensity,  and  made  the  band  seem  close  to  us. 

*  [On  June  26, 1863, 1  heard  a  rail-way  train  when  at  the  height  of  22,000  feet.— ED.] 


34  THE  ATMOSPHERE, 

When  sound  has  ceased,  there  still  continues  in  the  air  a  movement 
which  may  cause  to  vibrate  membranes  placed  to  receive  and  to  inter- 
pret these  impressions.  M.  Eegnault  has  measured  these  silent  waves ; 
he  has  determined  the  distance  traversed  both  by  the  sonorous  wave 
and  the  silent  wave  which  continues  after  the  former  has  ceased.  In  a 
gas-pipe,  twelve  inches  in  diameter,  a  pistol,  with  a  charge  of  fifteen 
grains  of  gunpowder,  was  heard  at  the  other  extremity,  6250  feet  off; 
and  when  the  pipe  was  closed  with  an  iron  plate,  the  echo  of  the  report 
was  perceptible  to  any  one  listening  attentively.  The  limit  of  the  so- 
norous wave  was  therefore,  in  this  instance,  12,600  feet;  that  of  siknt 
waves  is  much  greater. 

Air,  the  vehicle  of  sound,  is  at  the  same  time  the  vehicle  of  smells 
and  of  all  the  emanations  that  are  exhaled  from  the  terrestrial  surface. 
But  smells  are  due  not  only  to  the  vibratory  movement,  like  sound  and 
light.  Fourcroy  was  the  first  to  establish  the  fact  that  they  are  in  part 
caused  by  the  volatilization  of  vegetables  or  other  matter ;  that  smells 
are  caused  by  actual  molecules  suspended  in  the  air — material  particles, 
very  slender  and  volatilized  in  the  atmosphere.  But  the  matter  seems 
to  become  almost  intangible. 

Nothing  can  give  a  more  faithful  idea  of  the  divisibility  of  matter 
than  the  diffusion  of  smells.  Three-quarters  of  a  grain  of  musk  placed 
in  a  room  develop  a  very  strong  smell  in  it  for  a  considerable  time, 
without  the  musk  perceptibly  losing  weight,  and  the  box  containing 
the  musk  will  retain  the  perfume  almost  indefinitely.  Haller  states 
that  papers  perfumed  with  a  grain  of  ambergris  were  quite  odoriferous 
at  the  expiration  of  forty  years.  I  remember  purchasing  upon  the 
quay  in  Paris,  some  twelve  years  ago,  a  pamphlet  which  had  a  pro- 
nounced odor  of  musk  about  it.  It  had,  no  doubt,  been  there  many 
months,  exposed  to  the  sun,  the  wind,  and  the  rain.  Since  that  time  it 
has  remained  upon  a  library  shelf,  where  the  air  has  full  access  to  it, 
and  having  just  opened  its  pages,  I  find  it  as  fully  scented  as  ever. 

Smells  are  transported  by  the  air  to  great  distances.  A  dog  can  rec- 
ognize his  master's  approach  from  a  distance ;  and  it  is  asserted  that  at 
twenty-five  miles  from  the  coast  of  Ceylon  the  delicious  perfume  of  its 
balmy  forests  is  still  borne  upon  the  wind.  These  sweet  perfumes,  like 
the  harmony  and  the  activity  of  the  terrestrial  surface,  we  owe  to  the 
atmosphere. 


AERONAUTICAL  ASCENTS.  35 


CHAPTER  VII. 

AERONAUTICAL  ASCENTS. 

THE  air  being  a  fluid  possessing  weight,  analogous  to  water  in  regard 
to  the  principles  of  pressure,*  but,  as  we  have  seen,  very  much  lighter, 
an  instant's  reflection  will  suffice  to  show  that,  if  a  body  lighter  than 
air  be  placed  in  the  atmosphere,  it  will  rise  just  as  a  body  lighter  than 
water — such  as  wood  or  cork — will,  if  placed  at  the  bottom,  at  once 
ascend  to  the  surface,  because  of  its  less  specific  gravity. 

If  the  atmosphere  formed  a  homogeneous  ocean  above  the  surface  of 
the  globe,  equally  dense  throughout,  arid  terminated,  like  the  sea,  by  a 
defined  surface,  every  body  the  density  of  which  was  less  than  the 
density  of  this  aerial  ocean  would  rise,  when  left  to  itself,  by  the  as- 
censional force  of  a  pressure  dependent  on  the  difference  of  densities, 
and  would  remain  floating  upon  the  upper  surface  of  this  atmosphere. 
This  was  the  notion  of  several  of  the  predecessors  of  Montgolfier ; 
among  others,  of  the  worthy  Father  Galien,  in  his  fantastic  scheme  for 
aerial  navigation,  published  in  1755.  His  famous  ship  was  to  contain 
"  fifty-four  times  as  much  weight  as  Noah's  ark,"  its  dimensions  were 
to  be  equal  to  those  of  the  town  of  Avignon ;  for  the  hypothesis  of  this 
excellent  ecclesiastic  was  that  this  vast  iron  vessel  would  float  in  the 
atmosphere  in  virtue  of  the  same  principle  as  that  by  which  a  ship 
floats  upon  the  ocean.  But  as  the  density  of  the  atmospheric  strata 
diminishes  with  elevation,  all  objects  lighter  than  the  lower  strata 
mount  merely  to  the  region  the  density  of  which  is  such  that  the 
weight  of  the  body  is  equal  to  the  weight  of  the  volume  of  fluid  dis- 
placed. 

Archimedes  established  for  liquids  a  principle  which  we  can  apply 
with  precision  to  the  atmospheric  fluid,  enunciating  it  as  follows:  All 
bodies  situated  in  the  atmosphere  lose  a  portion  of  their  absolute 
weight,  equal  to  the  weight  of  the  air  which  they  displace. 

This  actual  loss  of  weight  in  the  air  is  proved  by  means  of  a  pair 
of  scales  specially  constructed  for  the  purpose,  as  the  name  indicates, 

*  [It  must  be  borne  in  mind  that  water  is  very  slightly  compressible  indeed ;  while  air  is 
an  elastic  fluid,  capable  of  almost  indefinite  compression  or  expansion. — ED.] 


86 


THE  ATMOSPHERE. 


Pig.  25.— The  Baroscope. 


of  seeing  the  weight-the  baroscope.     One  extremity  of  the  beam  has 
attached  to  it  a  hollow  copper  sphere;  the  other  end  carries  a  small 

piece  of  lead,  balancing  in  the  air  the 
copper  sphere.  If  this  apparatus  is 
placed  under  the  glass-receiver  of  an 
air-pump,  as  soon  as  a  vacuum  has 
been  created  the  balance  inclines  to 
the  side  of  the  sphere,  showing  that  in 
reality  it  weighs  more  than  the  mass 
of  lead  which  was  in  equilibrium  with 
it  when  in  the  air ;  or,  in  other  words, 
that  it  has  lost  in  the  air  a  portion 
of  its  weight,  because  its  volume  was 
larger  than  that  of  the  piece  of  lead. 
To  verify,  by  means  of  the  same  apparatus,  that  this  loss  is  just  equal 
to  the  weight  of  the  air  displaced,  the  volume  of  the  sphere  must  be 
measured,  and  if  it  holds  say  about  a  pint,  or  34 -6  cubic  inches,  the 
weight  of  this  volume  of  air  being  11-3  grains,  the  corresponding 
weight  must  be  attached  to  the  piece  of  lead,  and  the  equilibrium  will 
be  re-established  in  the  vacuum,  but  will  be  destroyed  upon  the  re- 
introduction  of  air. 

Let  us  note,  en  passant,  in  reference  to  this  subject,  that  when  any 
object  is  weighed  in  scales  it  is  never  its  exact  weight  which  is  ob- 
tained, but  its  apparent  weight.  To  get  at  the  actual  weight,  the  ob- 
ject must  be  weighed  in  vacuum.  This  is  a  source  of  continual  error 
which  is  rarely  taken  into  consideration.  But,  on  the  other  hand,  it 
may  be  asked,  what  is  the  real  weight  of  any  particular  body  ?  and  the 
reply  must  be,  there  is  no  such  thing.  It  is  a  purely  relative  matter, 
resulting  from  the  volume  and  density  of  the  planet  which  we  inhabit. 
A  pound  weight  does  not  constitute  an  absolute  quantity,  notwith- 
standing appearances  to  the  contrary.  The  proof  of  this  is,  that  if  a 
pound  weight  were  transported  to  the  surface  of  the  sun  it  would  weigh 
nearly  twenty-eight  pounds  ;*  whereas  it  would  weigh  two  pounds  and 
three  quarters,  nearly,  upon  the  surface  of  Jupiter,  and  only  one-sixth 
of  a  pound  at  the  moon !  And  even  without  going  so  far  as  this,  if  we 
imagine  our  atmosphere  gradually  becoming  denser  and  denser,  we 

*  [The  weighing  must,  of  course,  be  made  by  means  of  a  spring-balance,  or  other  balance 
of  the  same  kind.  If  a  certain  object  balances  a  pound  weight  on  the  earth  in  a  pair  of  scales, 
it  would  do  so  also  anywhere  else — on  the  sun,  moon,  etc. — ED.] 


AERONAUTICAL  ASCENTS.  87 

should,  in  that  case,  become  lighter;  or,  again,  if  the  earth  revolved 
seventeen  times  faster  than  it  does,  the  inhabitants  of  tropical  countries 
would  have  no  weight  at  all,  and  only  weigh  a  few  grains  in  the  lati- 
tude of  London  or  Paris.  This  may  serve  to  confirm  the  doctrine  of 
those  English  philosophers  who,  with  Berkeley  at  their  head,  argued 
that  the  only  real  fact  is,  that  there  is  nothing  real  in  the  world. 

But  let  us  return  to  the  weight  of  the  air.  A  balloon  is,  in  fact, 
merely  a  body  lighter  than  the  weight  of  the  air  which  it  displaces,  and 
which  consequently  rises  in  search  of  its  equilibrium  into  higher  re- 
gions of  less  density,  where  it  will  only  displace  a  volume  of  air  equal 
to  its  own  weight.  It  is  clear  that,  far  from  being  in  opposition  to  the 
laws  of  gravity,  the  ascent  of  balloons  is,  on  the  contrary,  a  special  con- 
firmation of  them. 

Whatever  may  be  the  substance  which  is  used  for  filling  a  globe  of 
silk  or  other  material,  if  the  whole  apparatus— the  gas  which  fills  the 
envelope,  the  car,  the  net  to  which  it  is  attached,  the  aeronauts,  etc. — 
weighs  less  than  the  air  which  it  displaces,  it  constitutes  by  that  very 
fact  an  aerostatical  machine,  and  rises  in  the  atmosphere. 

When  Montgolfier  launched,  for  the  first  time,  a  balloon  into  the  air, 
his  balloon  was  simply  inflated  with  hot  air.  The  density  of  air  heated 
up  to  122°  (Fahr.)  is  0-84,  that  of  air  at  32°  being  represented  by  1. 
The  density  at  212°,  the  temperature  of  boiling-water,  is  072,  giving 
scarcely  a  difference  of  one-third  for  the  ascensional  force.  The  den- 
sity of  pure  hydrogen  is  only  O07 ;  that  is,  one-fourteenth  of  that  of  air. 
The  density  of  carbureted  hydrogen  is  about  0'55;  that  is,  about  one- 
half  the  density  of  air.  The  latter  of  these  two  gases  is  generally  used 
for  filling  balloons. 

By  a  happy  coincidence  not  rare  in  the  history  of  science,  hydrogen 
gas  was  discovered  almost  simultaneously  with  the  invention  of  bal- 
loons. In  1782,  Cavallo  exhibited  before  audiences,  at  his  London 
lectures,  soap-bubbles  formed  of  hydrogen,  which  rose  by  their  less 
specific  gravity  up  to  the  ceiling  of  the  hall.  In  the  following  year 
(June  5,  1783)  Montgolfier  launched  the  first  aerostat.  With  a  little 
study  and  energy,  Cavallo  might  have  deprived  the  Annonay  manu- 
facturer of  the  immortality  of  his  invention. 

A  balloon  inflated  with  hot  air  is  still  often  called  a  Montgolfier  bal- 
loon, after  its  inventor.  A  balloon  inflated  with  gas  is  denominated 
a  gas-balloon,  and  often,  popularly,  an  air-balloon.  Gas  has  been 
adopted  almost  exclusively  since  its  first  trial,  which  was  made  at 


THE  ATMOSPHERE. 


Fig.  20.— Soap-babbles  Inflated  with  hydrogen. 

Paris,  on  the  27th  of  August,  1783,  by  M.  Charles,  Member  of  the 
Academy  of  Sciences,  and  the  Brothers  Robert. 

The  first  time  that  a  car  was  suspended  to  a  balloon  was  on  the  19th 
of  September,  1783,  in  presence  of  Louis  XVI.  and  Marie  Antoinette, 
at  Versailles ;  and  the  earliest  passengers  were  a  sheep,  a  cock,  and  a 
duck.  The  first  real  aerial  voyage  was  accomplished  on  the  21st  of 
October  following,  by  Pilatre  des  Hosiers  and  the  Marquis  d'Arlandes, 
who  rose,  by  means  of  a  fire-balloon,  from  the  Chateau  de  la  Muette 
(the  Bois  de  Boulogne),  and  made  their  descent  at  Montrouge  (on  the 
south  side  of  Paris),  after  having  crossed  the  capital. 

To  say  that  one/eefe  one's  self  being  carried  up  by  a  balloon  perhaps 
scarcely  gives  a  correct  idea  of  the  situation.  It  is  better  to  say,  sees 
one's  self  carried  up,  for  the  voyager  feels  no  kind  of  movement,  and 
the  earth  seems  to  him  to  be  descending. 

As  personal  impressions  are  unquestionably  those  the  recital  of 
which  comes  nearest  to  the  reality,  I  will  take  the  liberty  of  citing 
some.  My  first  ascent  took  place  on  Ascension-Day  (May  25)  in  1867. 
Eugene  Godard,  the  aeronaut,  having  verified  the  perfect  equilibrium 
of  the  balloon,  orders  the  four  assistants  to  let  slip  through  their  hands, 
without  losing  hold  of  them,  the  ropes  which  secured  the  car,  and  thus 
we  find  ourselves  a  few  yards  above  the  ground.  The  sky  is  clear,  the 


AERONAUTICAL  ASCENTS.  89 

wind  light,  and  the  balloon,  filled  with  hydrogen  gas,  becomes  impa- 
tient and  endeavors  to  rise.  Then,  taking  a  sack  of  ballast  in  his 
hand,  Godard  gives  the  word  to  ."  let  go,"  throwing  over  a  few  pounds 
of  sand,  and  the  aerostat  rises  with  majestic  ease. 

The  balloon  rises  in  an  oblique  curve,  caused  by  two  component 
forces — its  ascensional  power  on  the  one  hand,  and  the  velocity  of  the 
wind  on  the  other.  If,  as  is  proper  from  all  points  of  view,  we  take 
care  to  let  the  balloon  have  only  a  slight  ascensional  force,  the  most 
magnificent  of  panoramas  is  slowly  developed  before  the  charmed  gaze. 
If  we  wish  only  to  ascend  to  a  height  of  3000  to  4000  feet,  the  balloon 
is  allowed  to  move  horizontally  as  soon  as  i-t  reaches  an  atmospheric 
stratum  of  this  elevation,  whose  density -is  then  equal  to  that  of  the 
balloon.  For  higher  ascents,  the  balloon  is  lightened  by  throwing  out 
ballast. 

The  aeronaut,  the  meteorologist,  or  the  astronomer  who  thus  hovers 
in  the  air,  is  in  a  most  enviable  position  for  studying  the  atmosphere. 
Penetrating  into  the  very  midst  of  the  clouds,  traversing  them  to  de- 
termine the  light  and  heat  which  influence  them,  following  the  storm 
in  its  mysterious  formation,  studying  the  production  of  rain,  snow,  and 
the  hail,  transporting  himself,  in  fact,  into  the  very  regions  where  these 
phenomena  are  occurring,  it  is  there  alone  that  the  observer  is  really 
master  of  the  globe.  The  savant  may  in  vain  spend  years  by  his  fire- 
side in  forming  hypotheses  by  the  aid  of  books  and  apparatus ;  but  in 
this,  as  in  most  other  things,  the  surest  method  of  ascertaining  what  is 
going  on,  is  "  to  go  and  see  for  one's  self,"  as  the  old  proverb  has  it. 
And,  assuredly,  no  attempt  can  yield  more  fruitful  results. 

I  do  not  intend  to  revert  to  a  subject  which  was  largely  and  com- 
pletely dealt  with  in  1870  in  a  work  specially  devoted  thereto.  The 
purpose  of  this  chapter  is  not  to  record  my  travels  in  the  air;  the  scien- 
tific results  flowing  from  them  will  be  found  embodied  in  the  various 
explanations  which  compose  the  present  book.  It  was  merely  necessa- 
ry to  lay  down  the  general  theory  of  the  ascent  of  a  balloon  in  its  rela- 
tions to  the  study  of  the  atmosphere,  and  to  give  some  idea  of  the  effects 
of  the  higher  regions. 

If  aerial  travels  may  be  profitably  applied  to  the  study  of  the  forces 
at  work  in  the  atmosphere,  and  of  the  laws  which  preside  over  its  mul- 
tiform movements,  they  are  also  a  special  subject  of  interest  for  the  ob- 
server, and  open  for  him  an  exclusive  vista  of  vast  and  useful  contem- 
plation. Borne  into  the  fields  of  the  sky  by  the  invisible  breath  of  the 


go  THE  ATMOSPHERE. 

winds,  the  solitary  balloon  rises  above  the  earth,  and  the  traveler  views 
its  surface  as  a  map  stretched  out  on  a  boundless  plain  seen  with  all  the 
characteristics  of  its  local  topography.  Capitals  situated  on  the  banks 
of  rivers,  the  central  cities  of  provinces,  innumerable  villages  dissemi- 
nated over  the  country,  and  succeeding  each  other  in  hundreds  like  the 
little  chateaux  one  used  to  see  dotted  down  in  old-fashioned  maps,  hill- 
sides brown  with  the  vine,  furrows  golden  with  grain,  verdant  mead- 
ows, cragged  mountains  whose  tops  are  covered  with  sombre  forests, 
sparkling  streams  and  sinuous  rivers  running  to  the  distant  ocean— all 
the  charms,  soft  or  stern,  of  landscape  and  perspective  are  slowly  re- 
vealed to  the  delighted  gaze  of  the  aeronaut  who,  without  feeling  the 
slightest  movement,  hovers  as  in  a  dream  until  he  again  sets  foot  upon 
the  earth  that  he  has  been  contemplating  from  on  high.  A  less  power- 
ful impression,  but  of  a  similar  kind,  is  derived  from  a  mountain  ascent. 

The  purity  of  the  upper  air,  and  the  variation  in  atmospheric  press- 
ure, are  physical  elements  which  must  be  taken  into  account  in  order 
to  explain  the  benefit  of  a  sojourn  at  a  moderate  altitude.  The  peculiar 
action  which  may  be  exercised  upon  impressionable  organizations  by 
the  contemplation  of  mountains,  where  nature  has  bestowed  so  liberally 
that  mixture  of  the  gracious  and  the  terrible  which  tends  to  make  up 
the  picturesque,  is  undeniable.  J.  J.  Rousseau  says :  "  Every  one  must 
feel,  though  he  may  not  observe  it,  that  in  the  purer  and  more  subtle 
air  of  the  mountains  he  has  a  greater  facility  of  breathing,  more  nimble- 
ness  in  the  body,  more  serenity  of  mind;  the  pleasures  are  less  ardent 
there,  as  the  passions  are  more  subdued.  Meditation  assumes  a  certain 
tranquil  voluptuousness,  which  is  not  in  the  least  sensuous  or  bitter.  It 
seems  that,  as  we  rise  above  the  abode  of  man,  we  leave  all  terrestrial 
and  base  sentiments  behind,  and  as  we  approach  the  ethereal  regions, 
the  soul  gains  something  of  their  inalterable  purity.  We  become  grave 
without  being  melancholy,  placid  without  indolence,  content  to  live  and 
to  think.  I  doubt  whether  any  violent  agitation,  any  hysterical  affec- 
tion, could  hold  out  against  a  lengthened  sojourn  there ;  and  I  am  as- 
tonished that  a  bath  of  the  healthy  mountain  air  is  not  one  of  the  great- 
est medical  remedies." 

It  is,  however,  proper  to  state  that,  beyond  moderate  altitudes,  the 
human  organism  is  susceptible  of  a  deleterious  influence,  owing  to  the 
change  in  atmospheric  pressure,  the  dryness  of  the  air,  and  the  cold. 

The  physiological  uneasiness  and  disturbances  which  are  felt  at  great 
heights  have  long  been  ascertained  facts.  As  early  as  the  fifteenth  cen- 


AERONAUTICAL  ASCENTS.  91 

tury  they  were  observed  and  described  by  Da  Costa,  under  the  name  of 
mal  de  montagne.  Later,  all  mountain  explorers  in  the  Alps,  the  Andes, 
and  the  Himalayas,  as  well  as  aeronauts,  have  noted  these  singular  per- 
turbations of  organism,  and  have  published  theories  more  or  less  plau- 
sible in  explanation  of  them.  The  principal  cause  assigned  since  De 
Saussure  has  been  merely  the  rarefaction  of  the  air ;  but  by  what  series 
of  actions  and  reactions  does  this  rarefaction  affect  the  human  body  ? 
That  was  the  point  which  needed  elucidation. 

In  1804,  Gay-Lussac  and  Biot  rose  as  high  as  13,000  feet  in  a  balloon. 
Gay-Lussac's  pulse  went  up  from  62  to  80  a  minute ;  that  of  Biot  from 
79  to  111.  In  the  memorable  ascent  of  July  17, 1862,  Messrs.  Glaisher 
and  Coxwell  attained  the  enormous  elevation  of  37,000  feet.  Previous 
to  the  start,  Glaisher's  pulse  stood  at  76  beats  a  minute,  Mr.  Coxwell's 
at  74.  At  17,000  feet  the  pulse  of  the  former  was  at  84— of  the  latter, 
at  100;  at  19,000  feet  Glaisher's  hands  and  lips  were  quite  blue,  but 
not  his  face ;  at  21,000  feet  he  heard  his  heart  beating,  and  his  breath- 
ing was  becoming  oppressed ;  at  29,000  feet  he  became  senseless,  and 
only  returned  to  himself  when  the  balloon  had  come  down  again  to  the 
same  level ;  at  37,000  feet  the  aeronaut  could  no  longer  use  his  hands, 
and  was  obliged  to  pull  the  string  of  the  valve  with  his  teeth.  A  few 
minutes  later  he  would  have  swooned  away,  and  probably  lost  his  life; 
The  temperature  of  the  air  was  at  this  time  12°  below  zero.  In  aero- 
stats, however,  the  explorer  remains  motionless,  expending  little  or  none 
of  his  strength,  and  he  can  therefore  reach  a  greater  elevation  before 
feeling  the  disturbance  which  brings  to  a  halt  at  a  far  lower  level  the 
traveler  who  ascends  by  the  sole  strength  of  his  muscles  the  steep  sides 
of  a  mountain. 

De  Saussure,  in  his  ascent  of  Mont  Blanc  on  the  2d  of  August,  1787, 
has  given  an  account  of  the  uneasiness  which  his  companions  and  him- 
self began  to  experience  when  a  long  distance  from  the  summit.  Thus, 
at  13,000  feet,  upon  the  Petit-Plateau,  where  he  passed  the  night,  the 
hardy  guides  who  accompanied  him,  to  whom  the  few  hours'  previous 
marching  was  absolutely  child's  play,  had  only  removed  five  or  six 
spadefuls  of  snow  in  order  to  pitch  the  tent,  when  they  were  obliged 
to  give  in  and  take  a  rest,  while  several  felt  so  indisposed  that  they 
were  compelled  to  lie  upon  the  snow  to  prevent  themselves  from  faint- 
ing. "The  next  day,"  De  Saussure  tells  us,  "in  mounting  the  last 
ridge  which  leads  to  the  summit,  I  was  obliged  to  halt  for  breath  at 
every  fifteen  or  sixteen  paces,  generally  remaining  upright  and  leaning 


92  THE  ATMOSPHERE. 

on  my  stock;  but  on  more  than  one  occasion  I  had  to  lie  down,  as  I 
felt  an  absolute  need  of  repose.  If  I  attempted  to  surmount  the  feel- 
in  o1,  my  legs  refused  to  perform  their  functions;  I  had  an  initiatory 
feeling  of  faintness,  and  was  dazzled  in  a  way  quite  independent  of  the 
action  of  the  light,  for  the  double  crape  over  my  face  entirely  sheltered 
the  eyes.  As  I  saw  with  regret  the  time  which  I  had  intended  for  ex- 
periments upon  the  summit  slipping  away,  I  made  several  attempts 
to  shorten  these  intervals  of  rest.  I  tried,  for  instance,  a  momentary 
stoppage  every  four  or  five  paces,  instead  of  going  to  the  limit  of  my 
strength,  but  to  no  purpose,  as  at  the  end  of  the  fifteen  or  sixteen  paces 
I  was  obliged  to  rest  again  for  as  long  a  time  as  if  Lhad  done  them 
at  a  stretch ;  indeed,  the  uneasy  feeling  was  strongest  about  eight  or 
ten  seconds  after  a  stoppage.  The  only  thing  which  refreshed  me  and 
augmented  my  strength  was  the  fresh  wind  from  the  north.  When,  in 
mounting,  I  had  this  in  my  face,  and  could  swallow  it  down  in  gulps,  I 
could  take  twenty-five  or  twenty-six  paces  without  stopping." 

Bravais,  Martins,  and  Le  Pileur,  in  their  celebrated  expedition  to 
Mont  Blanc  in  1844,  experienced  and  investigated  the  same  phenome- 
na upon  the  Grand  Plateau.  In  clearing  the  tent,  which  was  half  filled 
with  snow,  the  guides  had  continually  to  stop  for  breath.  An  internal 
uneasiness,  according  to  Martins,  made  itself  apparent  in  many  different 
ways.  The  appetite  was  gone.  The  strongest,  biggest,  and  most  har- 
dy of  the  guides  fell  upon  the  snow,  and  was  nearly  in  a  fit  when  the 
doctor,  Le  Pileur,  felt  his  pulse.  On  nearing  the  summit,  Bravais  was 
anxious  to  see  how  far  he  could  go  without  a  rest ;  at  the  thirty-second 
step  he  was  obliged  to  stop  short. 

All  the  indispositions  felt  by  the  savans  of  whom  we  have  been 
speaking,  and  by  many  other  travelers,  at  great  elevations,  have  been 
classed  in  the  following  list: 

Breathing. — The  breathing  is  accelerated,  impeded,  laborious;  and 
there  is  a  feeling  of  extreme  dyspnoea  at  the  least  movement. 

Circulation. — The  great  majority  of  travelers  have  noticed  palpita- 
tions, quickening  of  the  pulse,  beating  of  the  carotids,  a  sensation  of 
plenitude  in  the  vessels,  and  sometimes  the  imminent  approach  of  suf- 
focation and  various  kinds  of  hemorrhage. 

Innervation.—'VQTy  painful  headache,  a  sometimes  irresistible  desire 
to  sleep,  dullness  of  the  senses,  loss  of  memory,  and  moral  prostration. 

Digestion.—  Thirst,  strong  desire  for  cooling  drinks,  dry  ness  of  the 
tongue,  distaste  for  solid  food,  nausea,  and  eructations. 


AERONAUTICAL  ASCENTS.  93 

Functions  of  Locomotion. — Pains  more  or  less  severe  in  the  knees  and 
legs;  walking  causing  great  fatigue  and  exhausting  all  strength. 

These  disturbances  are  not  regular,  they  do  not  all  come  on  at  once, 
and  evidently  depend  a  good  deal  upon  the  strength,  the  age,  the  hab- 
its, and  the  previous  actions  of  the  individual.  They  seem  to  have  a 
greater  effect  upon  Alpine  climbers  than  in  other  mountainous  regions. 
Thus,  at  the  Great  St.  Bernard,  the  monastery  of  which  has  an  altitude 
of  only  8117  feet,  most  of  the  monks  become  asthmatic.  They  are 
compelled  to  descend  frequently  into  the  valley  of  the  Khone  to  regain 
their  health,  and  at  the  end  of  ten  or  twelve  years'  service  to  quit  the 
monastery  for  good,  under  penalty  of  becoming  quite  infirm ;  and  yet, 
in  the  Andes  and  Thibet,  there  are  whole  cities  where  people  can  en- 
joy as  good  health  as  anywhere  else.  Boussingault  says,  that  "  when 
one  has  seen  the  activity  which  goes  on  in  towns  like  Bogota,  Micui- 
pampa,  Potosi,  etc.,  which  have  a  height  of  from  8500  feet  to  13,000 
feet ;  has  witnessed  the  strength  and  agility  of  the  toreadors  in  a  bull- 
fight at  Quito  (which  is  9541  feet);  when  one  has  seen  young  and  deli- 
cate women  dance  for  the  whole  night  long  in  localities  almost  as  lofty 
as  Mont  Blanc,  where  De  Saussure  had  scarcely  the  strength  to  read  his 
instruments,  and  where  the  vigorous  mountaineers  fainted;  when  one 
remembers  that  a  celebrated  combat,  that  of  Pichincha,  took  place  at 
a  height  as  great  as  that  of  Monte  Eosa  (15,000  feet),  it  will  be  admit- 
ted that  man  can  become  habituated  to  the  rarefied  air  of  the  highest 
mountains." 

The  same  writer  is  also  of  opinion  that  in  the  vast  fields  of  snow, 
the  discomfort  is  increased  by  an  emission  of  vitiated  air  under  the 
action  of  the  solar  rays,  and  he  bases  this  impression  upon  an  experi- 
ment of  De  Saussure,  who  found  the  air  near  the  surface  of  snow  to 
contain  less  oxygen  than  that  of  the  surrounding  atmosphere.  In  cer- 
tain hollows  and  inclosed  valleys  of  the  higher  part  of  Mont  Blanc — 
in  the  Corridor,  for  instance — people  generally  feel  so  unwell  in  trav- 
ersing it,  that  the  guides  long  thought  that  this  part  of  the  mountain 
was  impregnated  with  some  mephitic  exhalation.  Thus,  even  now, 
whenever  the  weather  permits,  people  ascend  by  the  Bosses  ridge, 
where  a  purer  air  prevents  the  physiological  disturbances  from  being 
so  intense. 

Notwithstanding  that  one  may  become  gradually  accustomed  to  the 
attenuated  air  of  high  elevations,  certain  animals  can  not  live  there. 
Thus  cats,  taken  up  to  the  altitude  of  13,000  feet,  invariably  succumb, 


g^  THE  ATMOSPHERE. 

after  having  been  subject  to  singular  attacks  of  tetanus,  of  gradually 
increasing  intensity;  and,  after  making  tremendous  leaps,  succumb 
from  fatigue,  and  die  in  convulsions. 

We  will  conclude  these  remarks  by  mentioning  that  the  highest  in- 
habited spot  in  the  world  is  the  Buddhist  cloister  of  Hanle  (Thibet), 
where  twenty  priests  live  at  the  enormous  height  of  16,500  feet. 
There  are  other  cloisters  built  at  a  nearly  equal  height  in  the  province 
of  Guari  Khorsum,  upon  the  banks  of  the  lakes  Monsaraour  and  Ba- 
kous,  and  they  are  inhabited  all  the  year  round.  In  these  equatorial 
regions  one  can  live  very  easily  for  ten  or  twelve  days  at  an  altitude 
of  18,000  feet,  but  not  for  a  longer  time.  The  Brothers  Schlagintweit, 
when  they  explored  the  glaciers  of  the  Ibi-Gamin  in  Thibet,  encamped 
and  passed  the  night,  with  eight  men  of  their  expedition,  from  the  13th 
to  the  23d  of  August,  1855,  at  these  exceptional  elevations,  which  are 
rarely  visited  by  a  human  being.  For  ten  days  their  encampments 
varied  from  18,000  to  21,000  feet ;  that  is  to  say,  the  greatest  altitude 
at  which  a  European  ever  passed  the  night  These  three  brothers 
succeeded,  on  August  19, 1856,  in  mounting  to  an  elevation  of  24,339 
feet_farther  than  man  has  ever  yet  reached.  At  first  they  suffered  a 
good  deal  when  they  got  to  17,000  feet ;  but,  after  a  few  days,  they  felt 
nothing  but  a  passing  uneasiness  even  at  19,000  feet.  It  is,  however, 
probable  that  a  prolonged  stay  at  this  altitude  would  have  produced 
ill  effects. 

Three  or  four  years  ago,  Professor  Tyndall,  in  order  to  take  scientific 
observations,  passed  the  whole  night  upon  the  summit  of  Moni  Blanc, 
sheltered  only  by  a  small  tent.  The  guides  who  accompanied  him  were 
so  unwell  that  the  next  morning  they  were  obliged  to  make  their  way 
downward  as  quickly  as  possible. 

A  year  or  two  ago,  M.  Lortel,  who  had  several  times  ascended  to 
14,000  feet  upon  Mont  Blanc  without  discomfort,  and  who  doubted 
whether  another  1600  feet  could  superinduce  the  symptoms  asserted, 
went  to  the  summit  to  judge  for  himself.  He  writes:  "I  am  now  con- 
vinced, and  am  compelled  to  admit,  de  visu  and  rather  at  my  expense, 
that  there  really  do  exist  causes  of  disturbance  at  this  height  which  af- 
fect a  person  who  ascends  so  far,  especially  if  he  is  in  motion,  in  this 
rarefied  air.  This  is  also  the  result  of  my  personal  observations ;  and 
I  have  satisfied  myself  that  it  is  much  less  hurtful  to  the  organic  func- 
tions to  rise  to  great  heights  when  sitting  still  in  a  car  than  by  climbing 
over  the  snows." 


AERONAUTICAL  ASCENTS.  95 

To  complete  our  atmospheric  panorama,  it  is  interesting  to  see  what 
are  the  highest  points  of  the  mountainous  peaks  upon  which  man  is  liv- 
ing, and  what  are  the  highest  points  of  the  mountain  chains  which  raise 
into  the  rarefied  atmosphere  their  silent  and  icy  peaks.  The  highest 
spots  of  the  earth  which  are  inhabited  are : 

The  Buddhist  cloister  of  Hanle  (Thibet) 16,532  feet. 

Cloisters  on  the  sides  of  the  Himalaya 14,764  to  16,404  " 

The  post-house  of  Apo  (Peru) 14,377  " 

The  post-house  of  Ancomarca  (do.) 14,206  " 

The  village  of  Tacora  (do.) 13,691  " 

The  town  of  Calamarca  (Bolivia) 13,651  " 

The  vineyard  of  Antisana  (Republic  of  Ecuador) 13,455  " 

The  town  of  Potosi  (Bolivia),  ancient  pop.  :  100,000 13,323  " 

The  town  of  Puno  (Peru) 12,871  " 

The  town  of  Oruro  (Bolivia) 12,455  " 

The  town  of  La  Paz  (do.) 12,225  " 

Quito,  capital  of  the  Ecuador  Eepublic,  is  situated  at  an  altitude  of 
9541  feet ;  La  Plata,  capital  of  Bolivia,  at  9331  feet ;  Santa  Fe  de  Bo- 
gota, at  8730  feet.  The  highest  inhabited  spot  in  Europe  is  the  Monas- 
tery of  Mount  St.  Bernard,  which  is  8117  feet  high. 

The  highest  passes  of  the  Alps  are — the  pass  of  Mount  Cervin,  11,188 
feet ;  the  Great  St.  Bernard,  8110  feet ;  the  Col  de  Seigne,  8074  feet ; 
and  the  Furka,  8002  feet.  The  highest  passes  in  the  Pyrenees  are — 
the  Port  d'Oo,  9843  feet ;  the  Port  Viel  d'Estaube,  8402  feet ;  and  the 
Port  de  Pinede,  8202  feet. 

The  highest  mountains  in  the  world  are : 

Asia :  The  Gaurisankar,  or  Mount  Everest  (Himalaya) 29,003  feet. 

The  Kanchinjinga  (Sikkim,  Himalaya) 28,156  " 

The  Dhaulagiri  (Nepaul,  do.       ) 26,825  " 

The  Juwahir  (Kemaon,  do.       ) 25,670  " 

Choomalari  (Thibet,  do.       ) 23,945  " 

America:  The  Aconcagua  (Chili) 22,422  " 

The  Sahama  (Peru) 22,349  " 

The  Chimborazo  (Republic  of  Ecuador) 21,424  " 

The  Sorota  (Bolivia) 21,283  " 

Africa:  The  Kilimanjaro 20,001  " 

Mount  Woso  (Ethiopia) 16,601  " 

Oceania:  The  Mownna-Roa,  volcano  (Sandwich  Isles) 15,874  " 

Europe:  Mont  Blanc 15,797  " 

Monte  Rosa 15,211  " 

The  birds,  of  course,  represent  the  population  of  the  very  highest 
altitudes.  In  the  Andes  the  condor,  in  the  Alps  the  eagle  and  the  vult- 


Q*  THE  ATMOSPHERE. 

t/O 

ure,  hover  above  the  topmost  peaks.  Fitted  for  the  longest  journeys, 
they  are  the  greatest  sailors  in  the  atmospheric  ocean,  just  as  the  petrels 
and  the  gigantic  sea-swallows  are  the  great  sailors  over  the  Atlantic. 
The  choucas  (a  kind  of  jackdaw),  with  its  intensely  black  plumage  and 
yellow  beak  and  red  legs,  does  not  rise  so  high  into  the  atmosphere, 
but  it  is  especially  the  bird  of  the  highest  peaks,  of  the  region  of  snows 
and  barren  cones.  It  is  met  with  at  the  summit  of  Monte  Eosa  and  at 
the  Col  du  Geant,  at  over  11,500  feet. 

There  are  also  birds  more  graceful  in  form  which  live  in  the  region 
of  hoar-frost,  and  lend  a  little  animation  to  those  bleak  and  unchanging 
landscapes.  The  snow-chaffinch  has  so  great  a  preference  for  this  cold 
region  that  he  rarely  descends  to  the  zone  of  the  woods.  The  accenteur 
of  the  Alps  also  follows  him  to  great  elevations,  preferring  the  stony 
and  barren  region  which  separates  the  zone  of  vegetation  from  that 
of  perpetual  snow,  and  both  of  these  birds  sometimes  soar  as  high  as 
11,000  to  15,000  feet  in  "pursuit  of  insects. 

The  engraving  (see  Fig.  27)  represents  the  principal  kinds  of  birds 
according  to  the  maximum  height  to  which  they  fly.  The  earth  has  its 
birds,  like  the  air.  Certain  kinds  never  use  their  wings  but  for  a  few 
moments,  when  it  is  impossible  for  them  to  move  along  the  ground; 
for  instance,  all  the  gallinaceous  kinds.  The  region  of  snow  has  its 
own  kind,  just  as  it  has  its  characteristic  sparrows.  The  ptarmigan,  or 
snow-hen,  is  met  with  in  Iceland  as  in  Switzerland.  It  soars  far  above 
the  everlasting  hoar-frosts,  and  is  so  fond  of  the  snow  that  at  the  ap- 
proach of  summer  it  mounts  farther  in  search  of  it,  plunging  into  it 
with  evident  delight.  A  few  lichens,  grains  brought  up  there  by  the 
air,  suffice  for  its  food.  It  looks  for  insects,  with  which  it  nourishes  its 
young. 

The  insects  are,  indeed,  the  only  animals  which  are  abundant  in  these 
bleak  regions — a  fresh  analogy  with  the  polar  countries.  It  is  also  the 
class  of  coleoptera  which  predominate  in  the  higher  Alpine  regions. 
They  attain  to  9800  feet  on  the  southern  slope,  and  to  7900  feet  on  the 
opposite  side.  Their  wings  are  so  short  that  they  scarcely  seem  to  have 
any ;  one  would  imagine  that  nature  had  intended  to  protect  them  from 
the  strong  currents  of  air  which  would  undoubtedly  carry  them  away 
if  their  wings  had  not  been,  so  to  speak,  reefed.  One  does  every  now 
and  then  encounter  other  insects,  neuroptera  and  butterflies,  which  the 
winds  have  taken  up  to  these  heights,  and  which  are  afterward  lost 
amidst  the  snows.  The  seas  of  ice  are  covered  with  victims  that  have 


Fig.  27. — Distribution  of  kinds  of  Birds  according  to  height  of  flight. 

Condor  (has  been  seen  as  high  as  9000  metres,  or  29,500  feet)  ;  2.  Griffon  ;  3.  Vulture ;  4.  Sarcoromphns  ;  5.  Eagle ;  6.  Urndn ;  7.  Kite ;  8.  Falcon  , 
9.  Sparrow-hawk ;  10.  Fly-bird ;  11.  Pigeon  ;  12.  Buzzard ;  13.  Swallow  ;  14.  Heron  ;  15.  Crane ;  16.  Duck  an.l  Swan  (found  in  lakes  at  an  altitude 
of  1800  metres, or  5900  feet) ;  17.  Crow;  18.  Lark;  19.  Quail;  20.  Parrot;  21.  Partridges  and  Pheasants ;  22.  Penguin. 

7 


AERONAUTICAL  ASCENTS.  99 

perished  in  this  way.  Nevertheless,  there  are  certain  kinds  which  ap- 
pear to  travel  freely  as  high  as  13,000  or  16,800  feet.  In  my  aerial 
voyages,  I  have  met  with  butterflies  at  heights  to  which  the  birds  of 
our  latitudes  do  not  ascend,  and  at  more  than  9800  feet  above  the 
ground.  Dr.  J.  D.  Hooker  noticed  some  at  Mount  Momay,  at  an  alti- 
tude of  more  than  17,700  feet.  Such  is  the  scale  of  animal  life  in  these 
Alpine  zones,  where  the  fauna  gradually  becomes  scarcer,  finally  giving 
way  to  solitude  and  desolation.  Beyond  the  last  stage  of  vegetation, 
beyond  the  extreme  region  attained  by  the  insect  and  mammifers,  all 
becomes  silent  and  uninhabited ;  yet  the  air  is  still  full  of  microscopic 
animalcules,  which  the  wind  raises  up  like  dust,  and  which  are  dissemi- 
nated to  an  unknown  height. 


BOOK  SECOND, 

LIGHT  AND  THE  OPTICAL"  PHENOMENA  OF  THE  AIR. 


THE  DAY.  103 


CHAPTER  I 

THE   DAY. 

As  the  atmosphere  is  the  organizer  of  life;  as  all  beings,  animal  and 
vegetable,  are  so  constituted  as  to  be  able  to  breathe  in  its  midst  and 
construct,  by  means  of  its  fluid  molecules,  the  solid  tissue  of  their  or- 
ganisms, we  must  now  turn  our  attention  with  admiration  to  the  atmos- 
phere, as  being  still  further  the  ornament  of  nature,  and  we  shall  see 
that  we  owe  to  it  not  only  the  picture,  but  also  the  frame. 

Whether  the  sky  be  clear  or  cloudy,  it  always  seems  to  us  to  have 
the  shape  of  an  elliptic  arch ;  far  from  having  the  form  of  a  circular 
arch,  it  always  seems  flattened  and  depressed  above  our  heads,  and 
gradually  to  become  farther  removed  toward  the  horizon.  Our  ances- 
tors imagined  that  this  blue  vault  was  really  what  the  eye  would  lead 
them  to  believe  it  to  be ;  but,  as  Voltaire  remarks,  this  is  about  as  rea- 
sonable as  if  a  silk- worm  took  his  web  for  the  limits  of  the  universe. 
The  Greek  astronomers  represented  it  as  formed  of  a  solid  crystal  sub- 
stance ;  and  so  recently  as  Copernicus,  a  large  number  of  astronomers 
thought  it  was  as  solid  as  plate-glass.  The  Latin  poets  placed  the  di- 
vinities of  Olympus  and  the  stately  mythological  court  upon  this  vault, 
above  the  planets  and  the  fixed  stars.  Previous  to  the  knowledge  that 
the  earth  was  moving  in  space,  and  that  space  is  everywhere,  theologi- 
ans had  installed  the  Trinity  in  the  empyrean,  the  angelic  hierarchy, 
the  saints,  and  all  the  heavenly  host.  ...  A  missionary  of  the  Middle 
Ages  even  tells  us  that,  in  one  of  his  voyages  in  search  of  the  terres- 
trial paradise,  he  reached  the  horizon  where  the  earth  and  the  heavens 
met,  and  that  he  discovered  a  certain  point  where  they  were  not  joined 
together,  and  where,  by  stooping,  he  passed  under  the  roof  of  the  heav- 
ens. .  .  .  And  yet  this  vault  has,  in  fact,  no  real  existence!  I  have 
myself  risen  higher  in  a  balloon  than  the  Greek  Olympus  was  sup- 
posed to  be  situated,  without  being  able  to  reach  this  limit,  which,  of 
course,  recedes  in  proportion  as  one  travels  in  pursuit  of  it — like  the 
apples  of  Tantalus. 

What,  then,  is  this  blue,  which  certainly  does  exist,  and  which  veils 
from  us  the  stars  during  the  day  ? 


-^04  THE  ATMOSPHERE. 

The  vault  which  we  behold  is  formed  by  the  atmospheric  strata 
which,  in  reflecting  the  light  that  emanates  from  the  sun,  interpose  be- 
tween space  and  ourselves  a  sort  of  fluid  veil,  which  varies  in  intensity 
and  height  with  the  density  of  the  aerial  zones.  The  illusion  referred 
to  above  took  a  long  time  to  dispel,  and  it  was  also  a  work  of  time  to 
make  it  known  that  the  shape  and  dimensions  of  the  celestial  vault 
change  with  the  constitution  of  the  atmosphere,  with  its  state  of  trans- 
parency and  its  degree  of  illumination.  One  part  of  the  celestial  rays 
sent  from  the  sun  to  our  planet  is  absorbed  by  the  air,  the  other  part 
is  reflected ;  the  air,  nevertheless,  does  not  act  equally  on  all  the  col- 
ored rays  of  which  white  light  is  composed,  but  acts  like  a  glass,  al- 
lowing the  rays  toward  the  red  end  of  the  solar  spectrum  to  pass  more 
readily  than  those  in  the  neighborhood  of  the  blue  end.  This  differ- 
ence is  only  perceptible  when  the  light  passes  through  a  great  thick- 
ness of  air.  De  Saussure  pointed  out  that  the  blue  color  of  the  sky 
was  due  to  the  reflection  of  light,  and  not  to  a  hue  peculiar  to  aerial  par- 
ticles. "If  the  air  were  blue,"  he  says,  "the  distant  mountains,  which 
are  covered  with  snow,  would  appear  blue  also,  which  is  not  the  case." 
An  experiment  made  by  Hassenfratz  also  proves  that  the  blue  ray  is 
more  reflected;  in  fact,  the  thicker  the  atmospheric  stratum  is  which 
a  ray  traverses,  the  more  do  the  blue  rays  disappear  to  give  place  to 
the  red ;  and  as,  when  the  sun  is  near  to  the  horizon,  the  ray  has  to 
traverse  a  greater  thickness  of  air,  the  sun  therefore  appears  red,  purple, 
or  yellow.  The  blue  rays  are  also  frequently  absent  in  rainbows  which 
make  their  appearance  just  before  sunset. 

We  shall  see  further  on  that  it  is  the  vapor  of  water  accumulated  in 
the  air  which  plays  the  principal  part  in  this  reflection  of  the  light,  to 
which  we  owe  the  azure  of  the  sky  and  the  brightness  of  day. 

Very  recently,  Professor  Tyndall  reproduced  the  blue  of  the  sky  and 
the  tint  of  the  clouds  in  an  experiment  at  the  Royal  Institution.  Va- 
por of  different  substances,  of  nitrite  of  butylene,  of  benzene,  and  of 
carbonic  sulphide,  is  introduced  into  a  glass  tube ;  a  succession  of  elec- 
tric sparks  is  then  passed  through  it,  and  the  condensation  and  rarefac- 
tion of  the  vapor  augmented  ad  libitum.  As  soon  as  the  vapors  em- 
ployed, no  matter  what  their  nature  is,  are  sufficiently  attenuated,  the 
reflection  of  the  light  first  manifests  itself  by  the  formation  of  a  blue 
like  that  of  the  sky.  There  is,  I  will  suppose,  in  the  tube  a  half  atmos- 
phere of  air  mixed  with  vapor,  and  another  half  atmosphere  of  air  that 
has  passed  through  hydrochloric  acid.  The  proportion  and  density  of 
the  gas  can,  of  course,  be  varied. 


THE  DAY.  105 

The  vaporish  cloud,  after  having  first  assumed  the  blue  tint,  becomes 
more  condensed  and  white,  and  as  it  thickens,  becomes  exactly  like  real 
cloud,  presenting,  as  regards  polarization,  the  same  variation  of  phe- 
nomena. 

The  atmospheric  air  is  one  of  the  most  transparent  bodies  known. 
When  it  is  not  charged  with  mist  or  obscured  by  other  bodies,  we  can 
see  objects  at  an  immense  distance,  and  mountains  only  disappear  from 
our  view  when  they  are  below  the  horizon ;  but,  in  spite  of  its  slight 
power  of  absorption,  the  air  is  not  completely  transparent ;  its  mole- 
cules absorb  a  portion  of  the  light  which  they  receive,  permit  the  pas- 
sage of  another  part,  and  reflect  the  third;  and  hence  it  is  that  they 
give  rise  to  what  appears  a  vault,  that  they  light  up  terrestrial  objects 
which  the  sun  does  not  reach  directly,  and  effect  an  imperceptible  tran- 
sition between  day  and  night. 

It  is  easy  to  convince  one's  self  of  the  decrease  in  the  intensity  of 
the  solar  light  during  its  passage  through  the  atmosphere  by  daily  ob- 
servations. If  an  object  situated  near  the  horizon  is  watched  for  sev- 
eral days  together,  it  will  be  seen  that  it  is  more  visible  at  one  time 
than  at  another.  The  distance  at  which  its  details  fade  out  of  sight  is 
at  one  moment  less  than  at  another,  as  may  be  proved  by  direct  meas- 
urement; the  transparency  of  the  air  can  be  even  expressed  numeric- 
ally, as  has  been  done  by  De  Saussure  through  the  instrumentality  of 
the  diaphanometer.  The  distance  at  which  objects  disappear  does  not 
depend  upon  the  angle  of  vision  alone,  but  also  upon  the  manner  of 
their  illumination,  and  the  contrast  which  their  color  offers  to  surround- 
ing objects.  This  explains  why  the  stars,  despite  their  small  diameter, 
are  so  visible  in  the  vault  of  heaven.  It  is  the  same  with  some  terres- 
trial objects.  It  is  difficult  to  distinguish  a  man,  as  he  stands  out  in 
the  fields,  as  against  dark  surfaces ;  but  he  is  very  easily  seen  if  he  is 
placed  upon  an  elevation  so  as  to  stand  out  against  the  clear  sky. 
Hence  the  optical  illusions  so  common  in  mountainous  countries. 

While  the  chain  of  the  Alps,  seen  from  the  plain  at  a  great  distance, 
is  visible  in  its  minute  details,  the  spectator  standing  upon  one  of  its 
peaks  can  distinguish  hardly  any  thing  in  the  plain.  From  the  Faul- 
horn,  for  instance,  it  is  easy  to  make  out  very  distinctly  the  chain  of 
the  high  Alps ;  but  every  thing  in  the  valley  below  is  dim  and  con- 
fused. The  summits  of  the  Pilate,  the  Black  Forest,  and  the  Yosges 
are  clearly  defined  at  a  great  distance,  whereas  nothing  can  be  distin- 
guished in  the  plain  between  the  Alps  and  the  Jura.  Any  one  who 


JQ6  THE  ATMOSPHERE. 

has  passed  a  few  months  amidst  the  lakes  and  mountains  of  Switzer- 
land must  have  noticed  the  same  variations  in  the  visibility  of  objects. 

To  measure  the  intensity  of  the  blue  color,  De  Saussure  invented  the 
cyanometer,  which  is  composed  simply  of  a  strip  of  paper  divided  into 
thirty  rectangles,  the  first  of  which  is  of  the  deepest  cobalt  blue,  while 
the  last  is  nearly  white,  the  intermediate  colors  offering  every  conceiv- 
able shade  between  dark  blue  and  white.  If  it  be  found  that  the  blue 
of  one  of  these  rectangles  is  identical  with  that  of  the  sky,  this  identi- 
ty is  then  represented  by  a  number  corresponding  to  one  of  the  rect- 
angles, and  all  that  remains  to  be  done  is  to  arrange  the  scale  of  the 
instrument. 

Humboldt  perfected  this  apparatus,  and  rendered  it  capable  of  giving 
very  precise  measurements  of  the  blue  tint. 

The  mere  contemplation  of  the  heavens  tells  us  that  their  color  is  not 
the  same  at  every  altitude,  being  generally  deeper  at  the  zenith,  and 
gradually  becoming  lighter  toward  the  horizon,  where  it  is  often  nearly 
white.  The  contrast  is  rendered  the  more  striking  by  the  use  of  the 
cyanometer.  Thus  it  will  be  found  that  sometimes  the  color  corre- 
sponds to  the  number  twenty-three  in  the  neighborhood  of  the  zenith, 
and  to  the  number  four  near  the  horizon.  But  the  color  of  the  same 
part  of  the  sky  also  changes  pretty  regularly  during  the  day,  as  it  be- 
comes darker  from  morning  until  noon,  and  lighter  again  from  noon 
until  evening.  In  our  climates  the  deepest  blue  is  when,  after  several 
days  of  rain,  the  wind  drives  away  the  clouds. 

The  color  of  the  sky  is  modified  by  the  combination  of  three  tints — 
the  blue,  which  is  reflected  by  the  aerial  particles;  the  black  of  infinite 
space;  and  the  white  of  the  vesicles  of  mists  and  snow-flakes  which 
float  at  the  high  elevations.  If  we  rise  sufficiently  high  in  the  atmos- 
phere, we  leave  a  part  of  the  vesicles  of  vapor  below  us.  Thus  the 
white  rays  reach  the  eye  in  a  lesser  proportion,  and,  the  sky  being  cov- 
ered with  fewer  particles  which  reflect  the  light,  its  color  becomes  of  a 
deeper  blue. 

The  nature  of  the  ground  also  plays  an  important  part  in  these  effects 
of  reflection  and  atmospheric  transparency. 

In  the  regions  where  there  are  vast  surfaces  devoid  of  vegetation,  as 
in  a  great  part  of  Africa,  the  air  is  very  dry,  and  loses  part  of  its  trans- 
parency, especially  in  consequence  of  the  dust  borne  by  the  winds  and 
the  absence  of  heavy  rain  to  cleanse  the  air.  In  the  other  parts  of  the 
intertropical  zone,  upon  the  Atlantic,  on  the  American  continent,  in  the 


THE  DAT.  107 

South  Sea  Islands,  and  in  certain  regions  of  India,  aqueous  vapor,  in  a 
state  of  transparent  gas,  is  abundantly  mixed  with  the  air ;  and  in  place 
of  the  grayish  hue  which  it  possesses  in  our  climates  and  in  sandy  des- 
erts, the  sky  presents  a  strongly -marked  tint  of  azure  blue,  which  spe- 
cially characterizes  the  regions  about  the  zenith,  and  sometimes  even 
the  sky  near  the  horizon. 

The  limiting  surface  of  the  atmosphere  being  parallel  to  that  of  the 
earth,  and  the  visible  portion  being  that  only  which  is  above  the  plane 
of  the  horizon,  it  is  clear  that  rays  of  light  reaching  the  eye  in  different 
directions  have  traversed  different  thicknesses  of  air.  If  the  sun  were 
at  the  zenith,  its  rays  would  pass  through  the  thinnest  stratum  of  air ; 
the  nearer  the  sun  approaches  the  horizon,  the  thicker  becomes  the  mass 
of  air  which  its  rays  have  to  pierce,  and  consequently  the  weaker  its 
rays  become.  The  light  of  the  sun  at  its  meridian  passage  is  dazzling, 
whereas  we  can  look  at  it  with  the  naked  eye  when  near  the  horizon ; 
and  for  the  same  reason  the  regions  situated  near  the  horizon  seem 
always  to  be  without  stars. 

The  color  of  the  sky  is  thus  explained  by  the  reflection  of  light  upon 
the  molecules  of  the  vapor  of  water  which  invisibly  pervades  the  air. 

How  are  we  now  to  explain  the  very  perceptible  shape  of  an  ellip- 
tical vault  which  the  sky  presents,  whether  cloudy  or  entirely  clear? 

This  may  be  explained  as  a  simple  effect  of  perspective. 

I  will  suppose  we  have  before  us  an  avenue  of  poplars,  all  of  the 
same  height.  Every  one  knows  that  this  height  will  apparently  de- 
crease with  distance,  and  that  the  top  of  the  trees  at  the  extreme  end  of 
the  avenue  will  appear  to  be  at  the  height  of  our  eyes. 

The  trees'  roots  are  upon  a  horizontal  surface,  because  we  ourselves 
are  upon  the  ground.  It  is  by  the  top  line  that  the  inclination  toward 
the  ground  operates.  If  we  were  in  the  upper  branches  of  the  nearest 
tree,  then  it  would  be  from  below  that  the  perspective  inclination  would 
operate.  The  same  train  of  reasoning  may  be  applied  to  the  clouds. 
Starting  from  those  which  are  vertically  above  our  heads,  they  succes- 
sively decline  in  height  according  to  their  distances  above  the  horizon. 

When  we  are  above  the  clouds  in  a  balloon,  they  no  longer  seem  to 
sink  toward  the  earth  like  a  vault,  but  to  extend  like  the  plane  surface 
of  an  immense  ocean  of  snow.  When  but  a  few  miles  above  them,  they 
describe  a  curve  in  the  contrary  direction.* 

*  [Having  been  led  theoretically  to  expect  such  a  phenomenon,  I  always,  when  some  miles 
above  the  clouds,  attentively  looked  for  its  appearance,  and  invariably  without  success.  It 


10g  THE  ATMOSPHERE. 

With  a  clear  sky,  the  surface  of  the  earth,  seen  from  a  great  height, 
is  hollow  underneath  the  car  of  a  balloon,  and  gradually  rises  around  up 
to  the  circular  horizon.  Far  from  appearing  convex,  as  might  be  ex- 
pected if  one  imagined  that  at  a  great  height  in  the  atmosphere  the 
spherical  shape  of  the  globe  would  be  recognized,  the  surface  of  the 
ground  is  hollowed  out  underneath  us,  rising  till  it  reaches  the  horizon, 
which  seems  always  to  be  on  a  level  with  the  eye. 

This  aspect  of  the  earth,  hollowed  out  like  a  basin,  surprised  me  very 
much  the  first  time  I  saw  it  from  a  balloon,  for  at  the  height  which  I 
had  attained  I  had  expected  to  see  it  convex. 

Thus  the  sinking  of  the  apparent  vault  of  the  sky  above  our  heads  is 
due  to  an  effect  of  perspective,  as  we  can  not  estimate  vertical  heights 
in  the  same  way  as  horizontal  lengths.  A  tree  forty-five  feet  high 
seems  much  longer  on  the  ground  than  when  standing.  A  tower  three 
hundred  feet  high  would  appear  far  more  if  laid  along  the  ground  than 
when  vertical.  Being  in  the  habit  of  walking  along  the  ground,  and 
not  of  soaring  into  the  air,  we  appreciate  lengths  at  their  true  estimate, 
whereas  heights  are  beyond  our  powers  of  direct  judgment. 

It  results  from  the  apparent  shape  of  the  celestial  vault  that  the  con- 
stellations seem  to  us  much  larger  toward  the  horizon  than  at  the  ze- 
nith (as,  for  instance,  the  Great  Bear  when  it  skirts  the  horizon,  and 
Orion  when  he  rises),  and  that  the  sun  and  the  moon  appear  to  have 
larger  disks  at  their  rising  and  setting  than  at  their  culminating  points. 
It  further  results  that  we  are  constantly  in  error  in  estimating  the 
height  of  stars  above  the  horizon.  A  star  which  is  at  45°  of  altitude — 
that  is,  just  half-way  between  the  horizon  and  the  zenith — seems  to  us 
much  higher ;  and  when  we  point  out  a  star  as  being  at  45°,  it  may 
happen  that  it  is  only  at  30°.* 

Modern  treatises  on  physics  and  meteorology  have  not  gone  into  this 

is  true  that,  the  dip  of  the  horizon  being  very  small,  objects  on  the  horizon  practically  ap- 
pear to  be  on  the  same  level  as  the  eye,  while  the  ground  underneath  of  course  seems  far  be- 
low, so  that,  in  this  sense,  the  appearance  of  the  earth  is  cup-shaped.  But,  in  point  of  fact, 
if  the  day  be  clear,  the  distance  of  the  horizon  is  so  much  greater  than  is  that  of  the  ground 
below,  that  the  effect  is  no  more  noticeable  than  it  is  from  the  top  of  a  hill.  If  the  air  be  not 
clear,  all  traces  of  the  appearance  are  of  course  absent.— ED.] 

*  [Most  people  imagine  they  are  looking  at  the  zenith  when  they  are  looking  at  a  point  10° 
or  20°  below  it,  and  on  this  account  their  estimates  of  heights  are  too  great.  As  regards  the 
shape  of  the  celestial  sphere,  it  may  be  remarked  that  the  distance  to  the  horizon  would  appear 
greater  than  to  the  zenith,  if  it  were  only  because  of  the  intervening  objects  which  occur  in  the 
former  case;  while,  looking  upward,  there  is  nothing  to  aid  the  eye  in  its  estimation.— ED.] 


THE  DAT.  109 

curious  question  of  the  aspect  of  the  sky.  I  find  it  discussed  in  certain 
works  of  the  seventeenth  and  eighteenth  centuries,  but  rather  from  a 
philosophical  point  of  view  than  in  its  purely  geometrical  aspect.  Af- 
ter a  long  dispute  between  Mallebranche  and  Eegis  upon  this  point, 
Eobert  Smith  examined  it  in  his  "Optics"  (1728),  and  concluded  that 
the  horizontal  diameter  of  the  celestial  vault  must  seem  to  us  six  times 
as  long  as  the  vertical  diameter.  He  is  of  opinion  that  this  is  due  to 
the  fact  that  "  our  view  does  not  extend  distinctly  to  the  point  at  which 
the  objects  form  an  angle  of  the  8000th  part  of  an  inch  in  our  eye,  so 
that  all  objects  seem  to  us  to  sink  under  the  horizon  at  a  distance  of 
25,000  yards." 

The  mathematician  Euler,  in  his  "Letters  to  a  German  Princess" 
(1762),  devotes  several  chapters  to  an  explanation  of  it,  which  may  be 
stated  in  a  few  words.  First,  the  light  of  the  stars  which  are  near  the 
horizon  is  much  weakened,  because  their  rays  have  a  greater  distance 
to  travel  through  our  lower  atmosphere  than  those  which  are  at  a  great- 
er height;  secondly,  being  less  luminous,  we  deem  them  to  be  farther 
off,  because  we  always  take  the  objects  which  are  most  clear  to  be  near- 
est to  us  (for  instance,  a  conflagration  at  night  seems  much  closer  to  us 
than  it  really  is) ;  thirdly,  this  apparent  distance  of  the  celestial  objects 
which  are  near  the  horizon  gives  rise  to  the  imaginary  elliptic  vault  of 
the  heavens. 

The  logical  arrangement  of  these  last  two  points  seems  the  inverse 
of  the  theory  explained  above,  yet  it  may  be  seen  that  these  two  facts 
do  not  follow  the  one  from  the  other,  but  are  simultaneous  in  our  ob- 
servation. Perspective  is  due  to  the  distance  and  to  the  diminution  in 
brightness,  and  it  gives  a  clear  explanation  of  the  apparent  shape  pre- 
sented by  the  atmospheric  strata,  and  the  variation  in  size  according  to 
the  elevation  above  the  horizon.  There  is,  so  to  speak,  a  double  effect 
of  geometrical  and  luminous  perspective. 

We  do  not  appreciate  the  beauty  or  the  practical  importance  of  the 
diffusion  of  light  by  the  air,  because  it  is  always  present  to  us.  A  so- 
journ of  a  few  hours  in  our  neighbor  the  moon  would  suffice  to  show 
us  the  enormous  difference  there  is  between  an  atmospheric  day  and 
one  without  air. 

As  Biot  remarked,  in  a  very  correct  simile,  the  air  is  around  the 
earth  a  sort  of  brilliant  veil,  which  multiplies  and  disperses  the  sunlight 
by  an  infinity  of  repercussions.  It  is  to  it  that  we  owe  the  light  which 
we  enjoy  when  the  sun  is  below  the  horizon.  After  the  latter  has  risen 


HO  THE  ATMOSPHERE. 

there  is  no  spot  so  secluded,  provided  the  air  has  access  to  it,  which 
does  not  receive  some  light,  although  the  sun's  rajs  may  not  reach  it 
directly.  If  the  atmosphere  did  not  exist,  each  point  of  the  terrestrial 
surface  would  only  receive  the  light  reaching  it  directly  from  the  sun. 

The  strange  effect  of  the  absence  of  the  atmosphere  would  be  far 
more  complete  and  striking  if  we  had  the  power  of  transporting  our- 
selves into  our  satellite.  Let  us  compare  the  cheerful  spectacle  that 
the  earth  presents,  partly  covered  with  its  humid  and  wavy  mantle, 
and  decked  with  flowers,  to  the  aspect  of  the  moon,  with  its  stony  or 
metallic  surface,  abounding  with  crevasses  and  vast  mountainous  des- 
erts, with  its  extinct  volcanoes  and  peaks  that  seem  like  gigantic  tombs, 
with  its  sky  invariably  black  and  shapeless,  in  which  reign,  day  and 
night,  stars  without  scintillation,  the  sun  and  the  earth.  There  day- 
time is,  so  to  speak,  nothing  but  night  lighted  up  by  a  rayless  sun. 
No  dawn  in  the  morning,  no  twilight  in  the  evening.  The  nights  are 
pitch-dark.  Those  parts  of  the  lunar  hemisphere  which  are  toward  us 
are  lighted  by  an  earth-light,  the  first  quarter  of  which  coincides  with 
sunset,  the  full  earth  with  midnight,  and  the  new  earth  with  sunrise.* 
In  day-time  the  solar  rays  are  lost  against  the  jagged  ridges,  the  sharp 
points  of  the  rocks,  or  the  steep  sides  of  their  abysses,  designing  here 
and  there  grotesque  shapes  against  the  angular  contours,  and  only  strik- 
ing the  surfaces  exposed  to  their  action  to  become  at  once  reflected  and 
lose  themselves  in  space — fantastic  shadows  standing  out  in  the  midst 
of  a  sepulchral  world.  Fig.  28  represents  a  landscape  taken  in  the 
moon,  in  the  centre  of  the  mountainous  region  of  Aristarchus.  There 
is  nothing  but  white  and  black.  The  rocks  reflect  passively  the  light 
of  the  sun ;  the  craters  remain  partially  wrapped  in  shade ;  fantastic 
steeples  seem  to  stand  out  like  phantoms  in  this  glacial  cemetery ;  the 
absence  of  the  atmosphere  leaves  the  black  space  of  the  starry  heaven 
perpetually  hanging  over  this  dismal  region,  to  which,  fortunately,  the 
earth  can  offer  no  sort  of  analogy. 

*  [The  moon  always  turns  the  same  face  to  the  earth ;  so  that  there  is  one-half  of  the 
moon's  surface  that  has  never  been  seen  from  the  earth.  The  words  one-half  must  not  be 
taken  quite  literally,  as,  owing  to  a  slight  oscillatory  motion  of  the  moon,  called  libration, 
we  sometimes  see  a  little  more  round  the  comer,  as  it  were,  than  at  other  times.  Speaking 
generally,  therefore,  an  inhabitant  of  the  moon,  if  he  saw  the  earth  at  all  (f.  e.,  was  on  the 
hemisphere  turned  toward  us),  would  always  see  it  in  the  same  position  in  the  sky  (and  in 
size  about  four  times  as  large  as  the  moon  appears  to  us).  The  statement  in  the  text  is  only 
true  for  a  spectator  placed  at  the  middle  point  of  the  visible  hemisphere  of  the  moon ;  the 
lunar  day  is  of  course  about  four  weeks.— ED.] 


Fig.  23.— Lunar  Day. 


EVENING. 


CHAPTER  II. 

*  EVENING. 

LIGHT,  that  imponderable  agent  which  enables  us  to  see  objects,  and 
which  by  its  qualities  illuminates  the  magnificent  atmospheric  world  in 
which  we  live,  gives  rise  to  an  ever-changing  series  of  effects.  The  at- 
mosphere not  only  bathes  the  landscape  with  light  by  reflection,  but 
also  decomposes  it  by  refraction,  and  gives  additional  variety  to  the 
beauties  of  the  earth  and  sky. 

When  a  ray  of  light  passes  from  one  transparent  medium  to  another, 
it  undergoes  a  deviation  caused  by  the  difference  of  density  of  the  two 
media.*  In  passing  from  air  to  water  the  ray  is  bent  toward  the  ver- 
tical, because  water  is  denser  than  air.  It  is  the  same  with  a  ray  which 
passes  from  a  higher  to  a  lower  stratum  of  air,  for,  as  we  have  seen,  the 
lower  strata  are  denser  than  those  above. 

If  a  ray  of  common  light  be  admitted  through  a  small  hole  in  a  dark- 
ened room,  and,  after  passing  through  a  glass  prism,  be  received  on  a 
screen,  it  will  be  seen  that  the  ray  of  white  light  has  been  decomposed 
by  refraction  through  the  prism  into  seven  colors  —  violet,  indigo, 
brown,  green,  yellow,  orange,  red — which  occupy  different  positions,  in 
the  above  order,  on  the  screen.  The  red  rays,  being  the  least  bent  from 
the  direction  of  the  original  ray,  are  said  to  be  least  refrangible,  and  the 
violet  rays,  which  form  the  other  end  of  the  spectrum,  are  said  to  be 
most  refrangible. 

In  refracting  light  the  air  produces  two  distinct  effects.  On  the  one 
hand,  it  causes  a  ray  of  light  which  has  its  origin  beyond  the  earth's  at- 

*  [M.  Flammarion  here  adds  the  sentence,  "A  stick  plunged  into  water  appears  bent  at  the 
surface  of  the  liquid,  and  the  immersed  portion  appears  more  nearly  vertical."  As  this  illus- 
tration of  the  effect  of  refraction  is  given  in  many  popular  works,  I  think  it  worth  while  to 
point  .out  its  inaccuracy.  A  ray  of  light  entering  a  denser  fluid  (the  surface  of  which  is  hori- 
zontal) is  bent  nearer  to  the  vertical ;  but  a  stick  is  not  a  ray  of  light,  and  in  no  way  resembles 
one.  The  immersed  portion  of  the  stick  is  seen  by  rays  that  have  been  refracted  at  the  surface 
of  the  water ;  and  it  easily  follows,  from  the  principles  of  optics,  that  the  part  under  water  ap- 
pears bent  from  (not  toward)  the  vertical.  This  any  one  can  verify  for  himself  experimentally. 
The  sentence  quoted  above  is  therefore  not  only  erroneous  in  theory,  but  also  incorrect  in  fact. 
The  apparent  bending  of  the  stick  is  only  indirectly  due  to  refraction. — ED.] 


H4.  THE  ATMOSPHERE. 

mosphere  to  become  bent  as  it  approaches  the  earth,  so  that  we  see  the 
sun,  moon,  planets,  comets,  and  the  stars,  as  if  they  were  higher  in  the 
heavens  than  they  really  are.  On  the  other  hand,  it  causes  a  more  or 
less  considerable  separation  between  the  various  rays  that  constitute 
white  light,  according  to  its  state  of  transparency  and  density. 

The  first  effect  mainly  produces  twilight ;  the  second  gives  that  soft, 
undulating  beauty  which  is  seen  in  the  serenity  of  the  evening. 

Eefraction  is  greater  or  less,  in  proportion  as  the  luminous  ray  trav- 
erses the  atmosphere  in  a  direction  more  or  less  inclined  to  the  ver- 
tical, being  greatest  for  horizontal  and  vanishing  for  vertical  rays.  As- 
tronomical observations  would  all  be  false  with  regard  to  the  positions 
of  objects  if  they  were  not  corrected  for  the  effect  of  refraction.  Thus, 


Fig.  29.— Atmospheric  refraction. 


for  instance,  the  star  A  is  seen  at  A';  the  star  B  at  B';  at  the  zenith 
alone  stars  are  where  they  appear  to  be,  there  being  no  alteration  in  the 
direction  of  the  ray  of  light  due  to  refraction.  To  make  these  neces- 
sary corrections,  tables  have  been  constructed  giving  refractions,  based 
upon  the  hypothesis  of  a  uniform  disposition  of  the  different  strata 
of  air  lying  one  above  the  other.  The  refracting  power  of  the  air  is 
determined  on  the  hypothesis  that  it  contains  only  oxygen  and  nitro- 
gen ;  but  we  have  seen  that  it  further  contains  from  four  to  six  parts  in 
10,000  of  carbonic  acid,  and  an  ever-varying  quantity  of  the  vapor  of 
water.  The  refracting  power  of  the  vapor  of  water  differs  so  little  from 
that  of  air  properly  so  called,  that  the  correction  depending  on  it  need 
not,  as  a  rule,  be  taken  into  the  calculation. 

To  calculate  the  amount  of  correction  to  be  applied  to  any  observa- 


EVENING. 


115 


tion,  it  is  only  necessary  to  note  at  the  time  the  temperature  of  the  air 
and  the  pressure  of  the  atmosphere  at  the  place  of  observation. 

To  illustrate  the  effect  of  refraction,  I  have  selected  from  a  table  of 
refractions  a  few  numbers,  at  different  zenith  distances.  They  show  to 
what  extent  objects  are  apparently  raised  by  its  influence : 

TABLE  OF  EEFRACTIONS. 


Distances 
from  the 
Zeuith. 

Refractions. 

Distances 
from  the 
Zenith. 

Refractions. 

90  deg. 

33  min.  47  sec. 

74  deg. 

3  min.  20  sec. 

89 

24 

22 

72 

2 

57 

88 

18 

23 

70 

2 

38 

87 

14 

28 

65 

2 

4 

86 

11 

48 

60 

1 

40 

85 

9 

54 

1     55 

1 

23 

84 

8 

30 

50 

1 

9 

83 

7 

25 

45 

0 

58 

82 

6 

34 

40 

* 

0 

48 

81 

5 

53 

30 

0 

33 

80 

5 

20 

20 

0 

21 

78 

4 

28 

10 

0 

10 

76 

3 

50 

0 

0 

0 

From  this  table  we  see  that  an  object  situated  just  upon  the  horizon 
is  raised  by  more  than  33',  or  about  -j-^j-  of  the  distance  from  the  hori- 
zon to  the  zenith.  Neither  the  sun  nor  the  moon  is  33'  in  diameter. 
When,  therefore,  they  appear  to  have  just  risen,  they  are  still  entirely 
below  the  horizon.  In  the  same  way,  the  sun  does  not  appear  to  begin 
to  set  until  after  sunset  has  actually  taken  place. 

It  follows  from  the.se  considerations  that  the  sun  may  be  seen  in  the 
west  and  the  moon  in  the  east  at  the  time  of  full  moon,  and  even  an 
eclipse  of  the  moon  may  be  visible  while  the  sun  is  still  above  the  hori- 
zon, although  the  earth  is  then  exactly  between  the  two  luminaries,  and 
the  latter  are  both,  astronomically  speaking,  below  the  horizon.  This 
is  due  to  refraction.  This  curious  circumstance  was  noted  during 
eclipses  of  the  moon  on  June  16, 1666,  and  May  26, 1668. 

Owing  to  the  same  cause,  the  sun  and  the  moon  seem  to  be  flattened 
both  at  their  rising  and  setting,  the  rays  proceeding  from  the  lower  edge 
of  the  luminary  being  more  refracted  than  those  proceeding  from  the 
upper,  so  that  the  apparent  vertical  diameter  is  diminished,  while  the 
horizontal  diameter  remains,  of  course,  unaltered.  The  length  of  the 
day  is  thus  increased,  and  that  of  the  night  decreased.  It  is  for  this  rea- 
son that  at  Paris  the  longest  day  of  the  year  is  sixteen  hours  seven  min- 
utes, and  the  shortest  eight  hours  eleven  minutes,  instead  of  being  fif- 


-Qg  THE  ATMOSPHERE. 

teen  hours  fifty-eight  minutes,  and  eight  hours  two  minutes.  We  see 
that  the  length  of  the  day  at  Paris  at  the  time  of  the  solstices  is  thus 
prolonged  by  nine  minutes,  and  by  seven  minutes  at  the  equinoxes. 
At  the  North  Pole  the  sun  seems  to  be  in  the  horizon,  not  when  it  ar- 
rives at  the  spring  equinox,  nor  when  its  angular  distance  from  the 
North  Pole  is  90°,  but  when  it  is  90°  33' ;  it  then  remains  visible  until, 
having  passed  to  the  autumnal  equinox,  its  polar  distance  has  again  be- 
come equal  to  90°  33'.  Care  must  always  be  taken  to  keep  account  of 
refraction  in  calculating  the  hours  of  sunrise  and  sunset. 

Twilight  is  that  light  which  remains  after  the  sun  has  set  or  which  is 
seen  before  sunrise.  The  duration  of  twilight  is,  in  many  respects,  a 
useful  element  to  be  acquainted  with.  It  depends  chiefly  upon  the  an- 
gle to  which  the  sun  has  descended  below  the  horizon ;  but  it  is  modi- 
fied by  several  circumstances,  the  chief  of  which  is  the  degree  of  clear- 
ness of  the  atmosphere.  The  direct  light  of  the  sun  at  the  time  of  sun- 
set reaches  to  the  west ;  as  the  sun  sinks,  its  boundary-line  rises,  and 
some  little  time  afterward  crosses  the  zenith,  when  civil  twilight  ends; 
the  planets  and  large  stars  then  become  visible  to  the  naked  eye.  The 
eastern  half  of  the  sky  being  thus  first  deprived  of  direct  solar  light, 
night  begins  there.  Afterward,  the  boundary -line  (the  crepuscular 
curve)  itself  disappears  in  the  west;  then  the  astronomical  twilight 
ceases  and  night  has  fully  set  in.  Twilight  begins  or  ends  when  the 
sun  is  at  a  certain  distance  below  the  horizon ;  this  distance  is  variable, 
depending  upon  the  state  of  the  atmosphere.  It  may  be  taken  that  civil 
twilight  ends  when  the  sun  is  about  8°  below  the  horizon,  and  that  as- 
tronomical twilight  ends  when  the  sun  is  about  18°  below  the  horizon. 
The  phenomena  of  twilight  are  hardly  known  in  tropical  climates ;  as 
soon  as  the  sun  has  descended  below  the  horizon,  darkness  sets  in  sud- 
denly. This  was  remarked  by  Bruce  at  Senegal,  where,  however,  the 
air  is  so  transparent  that  Yenus  may  sometimes  be  distinguished  at  mid- 
day, and  in  the  interior  of  Africa  night  succeeds  day  almost  immedi- 
ately after  sunset.  At  Cumana,  Humboldt  tells  us,  twilight  lasts  but  a 
very  few  minutes,  although  the  atmosphere  is  higher  under  the  tropics 
than  in  other  regions. 

The  following  tables  give  the  length  of  the  civil  and  astronomical 
twilight  in  France  for  the  various  seasons  and  for  the  fifteenth  day  of 
each  month.  By  adding  the  duration  of  twilight  to  the  hour  of  sunset, 
the  time  at  which  each  of  the  twilights  terminates  is  readily  obtained, 
and  subtracting  it  from  the  hour  of  sunrise,  the  times  of  their  com- 


EVENING'. 


117 


mencement  are  found.  France,  from  the  Pyrenees  to  Dunkirk,  is  with- 
in the  41st  and  42d  degrees  of  latitude.  It  will  be  seen  that,  even 
within  these  trifling  limits,  there  is  a  perceptible  difference.  The  short- 
est civil  twilights  take  place  on  the  29th  of  September  and  the  15th  of 
March,  the  longest  on  the  21st  of  June ;  the  shortest  astronomical  twi- 
lights fall  upon  the  7th  of  October  and  the  6th  of  March,  the  longest  on 
the  21st  of  June.  North  of  50°  latitude,  the  astronomical  twilight  con- 
tinues all  night  for  some  time  both  before  and  after  the  summer  solstice. 

TABLE  OF  THE  LENGTHS  OF  THE  LONGEST  AND  SHORTEST  DAYS. 


Latitude. 

Length  of  the  Day. 

The  longest  : 
June  21. 

The  shortest  : 
.     December  21. 

42  degrees. 
44 

15  hrs.  13  min. 
15    "    28    " 

9  hrs. 
8    "    47  min. 

46 

15    "    44     " 

8    "    30    " 

48       " 

16    "      2    " 

8    "    14    " 

50        " 

16    "    24     " 

7    "    55    " 

TABLE  OF  THE  DURATION  OF  CIVIL  TWILIGHT. 


Month. 

Latitude. 

42  deg. 

44  deg. 

46  d< 

36  m 
34 
33 
34 
38 
41 
39 
36 
33 
33 
35 
37 

g. 

48  deg. 

60  drg. 

34m 
32 
31 
32 
35 
37 
36 
33 
31 
31 
33 
34 

n. 

35  min. 
33    " 
32    " 
33    " 
36     ' 
39      ' 
38     ' 
34     ' 
32      ' 
32    " 
34    " 
36    " 

in. 
<  ! 

38m 
35 
34 
36 
40 
44 
42 
37 
34 
35 
37 
39 

n. 

40m 
37 
35 
36 
42 
46 
44 
39 
36 
36 
39 
41 

n. 

February  
March  

April 

May 

June   

Julv  

August  

October 

November  

TABLE  OF  THE  DURATION  OF  ASTRONOMICAL  TWILIGHT. 


Latitude. 

42  deg. 

44  deg. 

46  cleg. 

48  deg. 

50  deg. 

January  

31 

1     33 

1     36 

1     40 

1     45 

February  
March  

24 
24 

1     26 
1     26 

1     29 
1     29 

1     32 
1     33 

1     36 

1     37 

April     .  . 

33 

1     35 

1     39 

1     44 

1     50 

May  

46 

1     52 

2       1 

2     11 

2     26 

June  
•  July 

56 
48 

2       5 
1     54 

2     19 
2       4 

2    36 
2     14 

3     13 
2     31 

qt> 

1     37 

42 

47 

54 

September   .... 

24 

1     26 

30 

34 

38 

October  

23 

1     25 

29 

33 

36 

November  

30 

1     32 

39 

43 

December  

34 

1     36 

40 

45 

50 

-Qg  THE  ATMOSPHERE. 

In  warm  countries,  the  presence  of  humidity  in  the  air  not  only  gives 
to  the  sky  its  dark  azure  tint,  but  also  has  the  effect  of  modifying  the 
vital  power  of  the  solar  rays.  At  the  equator  it  adds  to  the  thousand 
other  wonders  of  nature  an  incomparably  beautiful  display  of  light  both 
at  sunrise  and  sunset  The  sunset,  in  particular,  affords  a  spectacle 
indescribably  magnificent — a  superiority  over  sunrise  attributable  to 
the  presence  of  moisture  in  the  air.  This  is  more  abundant  in  the 
evening,  after  the  heat  of  the  da}7,  than  in  the  morning,  when  it  is  par- 
tially condensed  into  dew  by  the  effect  of  the  cooler  temperature  of 
night. 

It  is  not  in  our  climate  that  the  finest  sunsets  are  seen.  The  celestial 
blue  of  distant  mountains,  the  rose  or  violet  tints  which  in  turn  tinge 
the  nearer  hills,  and  the  warm  tones  of  the  soil,  harmonize  in  a  mar- 
velous manner,  when  the  sun  disappears  below  the  horizon,  with  the 
gleaming  gold  of  the  west,  the  red  or  roseate  tints  that  crown  it  in  the 
sky,  the  dark  azure  of  the  zenith,  and  the  more  sombre  and  often,  in 
contrast  to  the  others,  greenish  hue  which  prevails  in  the  east.  In  the 
equinoctial  regions,  these  soft  and  delicate  tints,  joined  to  the  varied  as- 
pect of  the  earth's  configuration  and  the  richness  of  vegetation,  produce 
more  striking  effects  than  with  us.  Sometimes  light  and  roseate  clouds, 
fringed  with  a  coppery  red,  produce  peculiar  effects,  similar  to  certain 
sunsets  in  our  regions ;  but  whenever  the  sky  is  clear  the  shades  differ 
entirely  from  those  of  the  temperate  zone,  and  present  a  special  charac- 
ter. Sometimes,  too,  the  indentations  of  mountains  situated  below  the 
horizon,  or  invisible  clouds  intercepting  a  part  of  the  solar  rays  which, 
after  sunset,  still  reach  the  elevated  regions  of  the  atmosphere,  give  rise 
to  the  curious  phenomenon  of  crepuscular  rays.  Then  may  be  seen, 
starting  from  the  point  where  the  sun  has  disappeared,  a  series  of  rays, 
or  rather  of  diverging  "glories,"  which  sometimes  extend  as  far  as  90°, 
and  even  in  some  instances  are  prolonged  as  far  as  the  point  opposite 
to  the  sun.  "Upon  the  ocean,"  as  M.  Liais  remarks,  "when  the  sky, 
near  the  equator,  is  free  from  cloud  in  the  visible  part,  and  when  the 
diverging  rays  mingle  with  the  crepuscular  arcs,  the  play  of  light  as- 
sumes a  form  and  brilliance  which  defy  all  description  or  pictorial  illus- 
tration. How,  indeed,  is  it  possible  to  depict  completely  the  rosy  tints 
of  the  arc  fringed  by  the  crepuscular  rays  that  border  the  segment 
which  is  still  strongly  lighted  up  from  the  west,  the  segment  itself  being 
tinged  with  a  bright  gold  hue?  How,  above  all,  is  it  possible  to  de- 
scribe the  tint  of  an  inimitable  blue,  different  from  that  of  noonday,  and 


EVENING.  H9 

occupying  that  portion  of  the  sky  which  is  included  between  the  ordi- 
nary azure  and  the  crepuscular  arc? 

"  To  all  this  splendor  of  the  western  sky  must  be  added  the  descrip- 
tion of  its  fires  as  reflected  upon  the  surface  of  the  waters  agitated  by 
the  trade-wind,  the  dark  blue  color  of  the  sea  to  the  east,  the  white  foam 
of  the  wave,  which  sharply  defines  upon  this  gloomy  background  the 
pale  roseate  arc  of  the  eastern  sky,  and  the  sombre  and  greenish  seg- 
ment of  the  horizon." 

What  spectacle  can  be  more  sublime  than  a  sunset  at  sea?  We  have 
attempted  in  the  illustration  to  recall  this  beautiful  spectacle.  The  col- 
ored clouds  which  float  in  this  western  sky  are  cirro-cumuli,  which  will 
be  described  in  the  chapter  upon  the  Clouds. 

The  setting  sun  is  nearly  always  accompanied  by  these  cirro-cumuli 
clouds,  which  serve  to  display  those  aspects  of  the  sky  which  are  of  so 
remarkable  a  beauty  in  the  west.  In  consequence  of  the  curvature  of 
the  earth,  sea-clouds  which  are  sometimes  seen  from  Paris  are  more 
than  two  miles  above  the  ocean,  and  are  formed  of  ice  and  snow,  even 
in  the  month  of  July.  These  are  nearly  the  highest  clouds,  and  pro- 
duce the  varied  forms  of  mountains,  fishes,  animals,  and  other  fantastic 
shapes,  which  one  may  discern  of  an  evening  upon  a  bright  and  rich 
ground  of  every  tint  that  light  can  give. 

To  the  preceding  remarks  may  be  added  one  of  a  more  general  and 
curious  nature,  in  reference  to  the  influence  of  the  evening  light  in  the 
construction  of  cities.  Towns  grow  in  a  westward  direction.  Paris, 
the  cradle  of  which  was  the  lie  de  la  Cite,  has,  in  its  successive  aggran- 
dizements, constantly  extended  toward  the  west.  Two  thousand  years 
ago,  Paris  was  situated  on  the  north-east  slope  of  Mount  St.  Genevi&ve, 
where  the  arenas  have  recently  been  discovered.  Under  the  Merovin- 
gians it  commenced  its  descent  toward  the  west,  and  has  unceasingly 
progressed  in  that  direction  ever  since.  The  wealthy  classes  have  a 
pronounced  tendency  to  emigrate  westward,  leaving  the  eastern  districts 
for  the  laboring  populations.  This  remark  applies  not  only  to  Paris, 
but  to  most  great  cities— London,  Vienna,  Berlin,  St.  Petersburg,  Turin, 
Liege,  Toulouse,  Montpellier,  Caen,  and  even  Pompeii. 

Whence  arises  this  tendency  ?  A  fact  so  universal  can  not  be  due  to 
accident.  Is  it  the  stream  of  the  Seine  which  has  taken  Paris  westward 
in  its  wake?  Not  so,  for  the  Thames  flows  in  a  contrary  direction, 
while  London  has  none  the  less  extended  to  the  west  like  Paris.  Twelve 
years  ago,  Doctor  Junod  (Comptes-Rendus  of  the  Academy  of  Sciences 


12Q  THE  ATMOSPHERE. 

in  1858)  offered,  as  an  explanation  of  this  fact,  the  statement  that  the 
east  wind  is  that  which  raises  in  the  greatest  degree  the  barometrical 
column,  while  the  west  wind  lowers  it  the  most,  and  therefore  inundates 
the  eastern  part  of  a  town  with  deleterious  gases,  so  that  the  latter  has 
to  put  up  not  only  with  its  own  smoke  and  miasmas,  but  also  with  those 
coming  from  the  western  portion.  It  may,  in  fact,  be  admitted  that 
people  prefer  going  where  fresh  air  is  to  be  found,  and  in  the  direction 
from  which  the  wind  blows  most  frequently. 

But  the  wind  is  not  the  same  in  all  countries.  For  my  own  part,  I 
am  more  inclined  to  see  in  this  fact  an  evidence  of  the  attraction  of 
light.  And  the  suggestion  is  an  extremely  simple  one.  It  may  be  re- 
marked that  people,  as  a  rule,  take  their  promenade  of  an  evening,  and 
not  of  a  morning,  and  always,  or  nearly  always,  in  the  direction  of  sun- 
set. This  disposition  has  led  to  the  formation  of  gardens,  country 
houses,  and  places  of  public  resort,  and,  little  by  little,  the  wealthy  pop- 
ulation of  a  large  city  extends  in  this  direction- 


1'- 

X .-; 


THE  RAINBOW. 


CHAPTER  III. 

THE   RAINBOW. 

THE  general  action  of  light  in  nature  is  always  evident  to  our  eyes ; 
its  effects  in  the  atmosphere  are  of  very  different  kinds,  and  produce 
a  thousand  optical  phenomena,  always  curious,  often  fantastic,  but  all 
capable  of  explanation  in  these  days  by  physical  laws.  We  shall  de- 
vote the  following  chapters  to  the  examination  of  the  phenomena  that 
are  due  to  this  agent,  at  once  so  powerful  and  so  delicate. 

The  most  common  of  these  phenomena  is  the  rainbow,  and  the  ex- 
planation of  it  will  aid  us  in  understanding  the  others. 

There  are  few  persons  who  have  not  remarked  in  water  falling  from 
a  fountain  or  cascade  the  production  of  a  miniature  rainbow  analogous 
to  the  majestic  arch  which  crosses  the  sky.  Whenever  these  small 
rainbows  are  seen,  three  circumstances  will  be  observed  in  connection 
with  them:  first,  that  drops  of  water  must  be  present;  secondly,  that 
the  sun  must  be  shining ;  and,  thirdly,  that  the  observer  must  be  be- 
tween the  sun  and  the  water. 

These  three  conditions  in  regard  to  the  production  of  the  rainbow  will 
explain  the  phenomenon  in  which  the  Jewish  religion  saw  the  presence 
of  Jehovah,  and  the  Greek  mythology  the  auspicious  influence  of  the 
goddess  Iris.  In  order  to  see  a  rainbow  as  a  result  of  the  action  of  light, 
whether  on  artificial  rain  or  on 
the  drops  of  rain  falling  from 
the  clouds  in  the  atmosphere, 
the  spectator's  back  must  be  to 
the  sun.  In  this  position,  the 
solar  rays  which  shine  upon 
the  drops  of  water  are  reflected 
and  refracted  as  follows :  Let  us 
suppose  a  drop  of  water,  A  1 1', 
in  the  atmosphere.  A  solar  ray 
reaches  this  drop  at  I,  and  pass- 
es into  it,  being  deflected  from 

a    Straight   line   by   refraction,         Fig.  30.-Simple  reflection  of  rays  in  a  drop  of  rain. 


122  THE  ATMOSPHERE. 

inasmuch  as  the  ray  passes  into  a  medium  of  different  density.  Arriv- 
in»  at  A  on  the  surface  of  the  small  sphere  of  liquid  which  constitutes 
the  drop,  it  is  reflected  and  returns  in  the  direction  of  A  i',  being  refract- 
ed on  emergence  into  the  direction  i'  M. 

This  ray,  so  decomposed  by  refraction,  presents  all  the  colors  ar- 
ranged in  regular  order,  as  each  color  possesses  a  different  degree  of  re- 
frangibility.  The  inclination  increases  from  red  to  violet;  that  is  to 
say,  that  if  the  red  ray  from  a  particular  drop  reaches  the  eye,  the  other 
ravs  proceeding  from  the  same  drop  can  not  reach  it  too;  but  a  drop  at 
a  less  elevation  in  the  air  can  send  a  violet  ray  which  will  be  visible  at 
the  same  time.  Thus,  the  observer  sees,  in  the  direction  of  these  drops, 
a  red  hue  above  and  a  violet  hue  below.  The  intermediate  drops  simi- 
larly emit  rays  which,  when  seen  by  the  eye,  are  of  the  colors  included 
between  red  and  violet,  forming  a  solar  spectrum,  the  colors  of  which, 
starting  from  the  lowest  arc,  are  v iolet,  indigo,  blue,  green,  yellow,  orange, 
red. 

Let  us  now  imagine  a  conical  surface  passing  through  the  drop,  and 
having  for  axis  the  straight  line  drawn  from  the  eye  of  the  observer  to 
the  sun.  Every  drop  of  water  which  is  upon  this  surface  of  the  cone 
produces  the  same  effect,  so  that  there  is  a  mass  of  spectra  forming  a 
circular  band,  in  which  the  simple  colors  succeed  each  other  in  the  or- 
der indicated,  the  violet,  a  (see  Fig.  33,  p.  124),  being  inside,  and  the  red, 
b,  outside. 

The  phenomenon  continues  as  long  as  the  drops  of  water  go  on  fall- 
ing in  the  same  region  of  space,  the  luminous  appearance  being  inces- 
santly renewed  by  the  falling  of  the  drops,  so  that  the  arch  appears  per- 
manent while  the  rain  lasts. 

Calculation  has  shown  that  the  angle  of  the  cone  of  the  red  rays  is 
42°  20',  and  that  of  the  violet  rays  40°  30'.  This  is,  therefore,  the  dis- 
tance from  the  arc  to  the  centre  or  the  point  of  the  sky  on  which  the 
shadow  of  the  head  of  the  spectator,  p  (see  Fig.  33),  would  be  cast.  The 
diameter,  H  H'  (see  Fig.  33),  of  the  whole  arc  subtends  an  angle  of  about 
84°,  the  width  of  the  arc  being  2°,  or  nearly  four  times  the  apparent 
diameter  of  the  sun. 

The  rainbow,  therefore,  demonstrates  the  existence  of  small  spheres 
of  liquid  water,  falling  as  rain  in  the  midst  of  the  atmosphere.  The 
arch  is  more  brilliant  as  their  size  increases.  They  must  be  much 
larger  than  those  which  form  the  clouds  for  the  eye  to  be  able  to  distin- 
guish the  colors,  and  that  is  the  reason  why  mists  and  clouds  do  not 


THE  RAINBOW.  123 

produce  any  rainbow.  Knowing  that  the  rainbow  is  caused  by  the  re- 
fraction of  the  sun's  rays  through  drops  of  rain  as  they  fall,  we  may  de- 
duce therefrom  not  only  the  size  of  this  arch,  but  also  the  conditions 
without  which  it  could  not  exist.  If  the  sun  were  on  the  horizon,  the 
shadow  of  the  spectator's  head  would  be  cast  there  also,  and  as  the  axis 
of  the  cone  would  be  horizontal,  it  follows  that  we  should  see  a  semi- 
circle of  an  apparent  radius  of  41°. '  As  the  sun  rises,  the  axis  of  the 
cone  is  inclined,  and  the  arch  becomes  smaller;  and  finally,  when  the 
sun  reaches  a  height  of  41°,  the  axis  of  the  cone  forms  the  same  angle 
with  the  plane  of  the  horizon,  and  the  top  of  the  arch  just  touches  this 


Fig.  31.— Formation  of  the  rainbow. 

latter  plane.  If  the  sun  were  still  higher,  the  arch  would  be  projected 
upon  the  ground.  The  phenomenon  is  rarely  visible  under  this  last  con- 
dition. The  secondary  rainbow,  of  which  I  am  about  to  speak,  disap- 
pears when  the  sun  reaches  an  altitude  of  52°,  for  which  reason  a  rain- 
bow can  not  be  seen  at  noon  in  summer.  The  observer  standing  upon 
the  earth  can,  therefore,  never  see  more  than  half  a  circumference  (viz., 
when  the  sun  is  on  the  horizon) ;  and,  as  a  rule,  the  arch  is  only  100° 
to  150°  in  length.  When  the  earth  does  not  stand  in  the  way  of  the 
production  of  the  lower  part,  more  than  a  semi-circumference,  and  even 
a  whole  circumference,  may  be  seen.  This  occurred  to  me  once  in  a 


124:  THE  ATMOSPHERE. 

balloon ;  and  by  a  curious  coincidence  (the  upper  part  being  concealed), 

I  saw  a  rainbow  upside  down,  in 
which  the  violet  color  was  inside. 
A  second  arch,  in  which  the 
colors  appear  in  an  inverse  order 
to  those  in  the  rainbow  described 
above,  is  frequently  remarked. 
This  second  arch  is  explained  by 
a  double  reflection,  s  I  A  B  i'  M  (see 
Fig.  32)  and  sVo,  s'b'o  (see  Fig. 

Fig.  32.-Donble  reflection  of  rays  in  a  drop  of  rain.    g3)        jn  ^  ^^  the  deyiations 

of  the  rays  after  they  emerge  from  the  liquid  sphere  are  51°  for  the  red 
rays,  and  54°  for  the  violet  rays.  This  secondary  arch  is  always  paler 
than  the  first 

The  zone  comprised  between  the  principal  and  the  secondary  arch  is 
generally  darker  than  the  rest  of  the  sky,  and  appears  to  me,  after  nu- 
merous observations,  to  be  a  region  of  absorption  for  the  luminous  rays. 

It  is  ascertained  that  a 
larger  number  of  reflections 
may  be  produced,  and  that 
other  arches,  more  and  more 
pale  in  hue,  may  exist.  But 
the  diffused  light  prevents 
them  from  being  seen.  How- 
ever, a  third  has  been  seen, 
at  40°  from  the  sun.  By 
causing  the  solar  rays  to  fall 
upon  a  jet  of  water  in  a  dark 
place,  as  many  as  seventeen 
rainbows  have  been  counted. 

It  may  happen  that  the 
sun  is  reflected  toward  a 
cloud  by  the  surface  of  a 
piece  of  still  water;  and 
then  this  reflection  will  also 
give  rise  to  a  rainbow.  It  has  been  found  that  in  this  case  the  rain- 
bow must  cut  the  arch  formed  by  the  direct  rays  at  a  height  dependent 
upon  that  of  the  sun.  If  the  two  phenomena  produce  a  secondary  arch, 
the  four  curves  intertwined  form  a  very  beautiful  spectacle.  A  case  in 


Fig.  33 — Theory  of  the  two  arches  of  a  rainbow. 


THE  RAINBOW. 


125 


which  they  were  quite  complete  and  perfectly  distinct  is  cited  by 
Monge.  Halley  observed  three  arches,  one  of  which  was  formed  by  the 
rays  reflected  upon  a  river.  This  arch  first  intersected  the  exterior 
arch  so  as  to  divide  it  into  three  equal  parts.  When  the  sun  sunk  to- 
ward the  horizon,  the  points  of  meeting  were  drawn  close  together. 
There  soon  was  seen  but  one  single  arch,  and  as  the  colors  were  in  in- 
verse order,  pure  white  was  formed  by  the  superposition  of  the  two  se- 
ries. The  sun,  too,  may  produce,  after  being  reflected  upon  a  piece  of 
water,  a  complete  circle,  the  upper  part  of  which  being  sometimes  invis- 


Pig.  34.— Triple  rainbow. 

ible,  gives  rise  to  the  singular  phenomenon  of  a  rainbow  upside  down. 
The  Academicians  dispatched  to  the  polar  regions  to  measure  an  arc  of 
the  meridian,  observed  upon  the  Ketima  Mountain,  on  July  17, 1736,  a 
triple  rainbow  analogous  to  that  of  which  Halley  speaks.  In  the  lower 
bow  the  violet  was  underneath,  the  red  outside  as  usual :  this  was  the 
principal  arch.  The  second,  which  was  parallel  to  it,  was  the  secondary 
arch.  In  this  the  red  was  underneath  and  the  violet  at  the  top.  The 
third  arch,  starting  from  the  extremities  of  the  first,  crossed  the  second, 


-^26  THE  ATMOSPHERE. 

and  had,  like  the  principal  one,  the  violet  inside  and  the  red  outside. 
This  is  the  phenomenon  drawn  in  Fig.  34. 

Seeing,  then,  that  the  rainbow  is  due  to  the  refraction  and  reflection 
of  the  solar  rays  upon  little  drops  of  water  falling  in  the  air,  it  is  easy 
to  conceive  that  moonlight  may  cause  an  analogous  appearance,  though 
less  intense ;  and  this  indeed  is  the  case,  though  a  lunar  rainbow  is  not 
very  common.  The  illustration  represents  a  lunar  rainbow  which  I 
had  an  opportunity  of  remarking  one  spring  evening  at  Compiegne. 

Many  observers  have  remarked  and  described  this  nocturnal  rain- 
bow. I  gather  from  the  writings  of  Americ  Vespuce  (1501)  that  he 
had  several  times  observed  "the  iris  at  night."  He  considers  that  the 
red  of  the  arch  is  due  to  fire,  the  green  to  the  earth,  the  white  to  the 
air,  and  the  blue  to  the  water;  and,  he  adds,  "this  sign  will  cease  to  ap- 
pear when  the  elements  are  used  up,  forty  years  before  the  end  of  the 
world." 

I  notice  in  an  ancient  treatise  on  meteorology  (that  of  P.  Cotte)  that, 
in  addition  to  the  ordinary  rainbow,  the  secondary  rainbow,  the  reflect- 
ed arches,  and  the  lunar  rainbow,  there  has  been  mentioned  yet  another 
optical  effect,  called  the  "  marine  rainbow,"  formed  upon  the  surface  of 
the  sea,  and  composed  of  a  large  number  of  zones.  It  sometimes  ap- 
pears upon  wet  meadows  lying  opposite  to  the  sun.  This  fifth  aspect  is 
a  kind  of  anthelion,  which  I  will  allude  to  in  the  next  chapter.  The 
name  of  "white  rainbow"  has  also  been  given  to  the  anthelical  circle, 
which  will  also  be  considered  in  the  same  chapter. 

Lastly,  there  are  sometimes  seen  colored  bands  below  the  violet  of 
the  ordinary  rainbow,  which  appear  to  belong  to  an  arch  lying  over  the 
first.  This  arch  then  takes  the  name  of  supernumerary  arch,  and  is  due 
to  very  complex  effects  of  interference  of  light,  explainable  on  the  un- 
dulatory  theory. 

The  first  person  who  attempted  to  explain  the  phenomenon  of  the 
rainbow  by  the  reflection  of  light  upon  the.interior  of  the  drops  of  rain 
was  a  German  monk  of  the  name  of  Theodoric ;  the  second  an  arch- 
bishop, A.  De  Dominis  (1611).  But  the  true  theory  was  first  given  by 
Descartes,  with  the  exception  of  the  separation  of  colors,  which  was  only 
determined  by  the  discovery  of  Newton  as  to  the  unequal  refrangibility 
of  the  rays  of  the  solar  spectrum. 


A    Mori*  piavr' 


chromolitk' 


LUNAR    RAINBOW   SEEN'  AT   COMPiEGNE 


ANTHELIA. 


127 


CHAPTER  IV. 
ANTHELIA:   SPECTRE-SHADOWS  UPON  MOUNTAINS — THE  ULLOA 

CIRCLE — CIRCLE  SEEN  FROM  A  BALLOON. 

TREATISES  on  meteorology  have  not,  up  to  the  present  day,  classified 
with  sufficient  regularity  the  diverse  optical  phenomena  of  the  air. 
Some  of  these  phenomena  have,  however,  been  seen  but  rarely,  and 
have  not  been  sufficiently  studied  to  admit  of  their  classification.  We 
have  examined  the  common  phenomenon  of  the  rainbow,  and  we  have 
seen  that  it  is  due  to  the  refraction  and  reflection  of  light  on  drops  of 
water,  and  that  it  is  seen  upon  the  opposite  side  of  the  sky  to  the  sun  in 
day-time  or  the  moon  at  night.  We  are  now  about  to  consider  an  order 
of  phenomena  which  are  of  rarer  occurrence,  but  which  have  this  prop- 
erty in  common  with  the  rainbow,  viz.,  that  they  take  place  also  upon 
the  side  of  the  sky  opposite  to  the  sun.  These  different  optical  effects 
are  classed  together  under  the  name  of  anihelia  (from  avOt,  opposite  to, 
and  rjAioc,  the  sun).  The  optical  phenomena  which  occur  on  the  same 
side  as,  or  around  the  sun,  such  as  halos,  parhelia,  etc.,  will  form  the 
subject  of  the  next  chapter. 

Before  coming  to  the  anthelia,  properly  so  called,  or  to  the  colored 
rings  which  appear  around  a  shadow,  it  is  as  well  first  to  note  the  effects 
produced  on  the  clouds  and  mists  that  are  facing  the  sun  when  it  rises 
or  sets. 

Upon  high  mountains,  the  shadow  of  the  mountain  is  often  seen 
thrown  either  upon  the  surface  of  the  lower  mists  or  upon  the  neigh- 
boring mountains,  and  projected  opposite  to  the  sun  almost  horizon- 
tally. I  once  saw  the  shadow  of  the  Righi  very  distinctly  traced  upon 
Mount  Pilate,  which  is  situated  to  the  west  of  the  Righi,  on  the  other 
side  of  the  Lake  of  Lucerne.  This  phenomenon  occurs  a  few  minutes 
after  sunrise,  and  the  triangular  form  of  Righi  is  delineated  in  a  shape 
very  easy  to  recognize. 

The  shadow  of  Mont  Blanc  is  discerned  more  easily  at  sunset.  MM. 
Bravais  and  Martins,  in  one  of  their  scientific  ascents,  noticed  it  under 
specially  favorable  circumstances,  the  shadow  being  thrown  upon  the 
snow-covered  mountains,  and  gradually  rising  in  the  atmosphere  until  it 


J28  THE  ATMOSPHERE. 

reached  a  height  of  1°,  still  remaining  quite  visible.  The  air  above  the 
cone  of  the  shadow  was  tinted  with  that  rosy  purple  which  is  seen,  in  a 
fine  sunset,  coloring  the  lofty  peaks.  "Imagine,"  says  Bravais,  "the 
other  mountains  also  projecting,  at  the  same  moment,  their  shadows  into 
the  atmosphere,  the  lower  parts  dark  and  slightly  greenish,  and  above 
each  of  these  shadows  the  rosy  purple  surface,  with  the  deeper  rose  of 
the  belt  which  separates  it  from  them ;  add  to  this  the  regular  contour 
of  the  cones  of  the  shadow,  principally  at  their  upper  edge,  and  lastly, 
the  laws  of  perspective  causing  all  these  lines  to  converge  the  one  to  the 
other  toward  the  very  summit  of  the  shadow  of  Mont  Blanc ;  that  is  to 
say,  to  the  point  of  the  sky  where  the  shadows  of  our  own  selves  were ; 
and  even  then  one  will  have  but  a  faint  idea  of  the  richness  of  the  me- 
teorological phenomenon  displayed  before  our  eyes  for  a  few  instants. 
It  seemed  as  though  an  invisible  being  was  seated  upon  a  throne  sur- 
rounded by  fire,  and  that  angels  with  glittering  wings  were  kneeling 
before  him  in  adoration." 

Among  the  natural  phenomena  which  now  attract  our  attention,  but 
fail  to  excite  our  surprise,  there  are  some  which  possess  the  characteris- 
tics of  a  supernatural  intervention.  The  names  which  they  have  re- 
ceived still  bear  witness  to  the  terror  which  they  once  inspired ;  and 
even  to-day,  when  science  has  stripped  them  of  their  marvelous  origin, 
and  explained  the  causes  of  their  production,  these  phenomena  have  re- 
tained a  part  of  their  primitive  importance,  and  are  welcomed  by  the 
savant  with  as  much  interest  as  when  they  were  attributed  to  divine 
agency.  Out  of  a  large  and  very  diverse  number,  I  will  first  select  the 
Spectre  of  the  Brocken. 

The  Brocken  is  the  highest  mountain  in  the  picturesque  Hartz  chain, 
running  through  Hanover,  being  3300  feet  above  the  level  of  the  sea. 

One  of  the  best  descriptions  of  this  phenomenon  is  given  by  the  trav- 
eller Hane,  who  witnessed  it  on  the  23d  of  May,  1797.  After  having 
ascended  no  less  than  thirty  times  to  the  summit,  he  had  the  good  for- 
tune at  last  to  contemplate  the  object  of  his  curiosity.  The  sun  rose  at 
about  four  o'clock,  the  weather  being  fine,  and  the  wind  driving  off  to 
the  west  the  transparent  vapors  which  had  not  yet  had  time  to  be  con- 
densed into  clouds.  About  a  quarter  past,  four,  Hane  saw  in  this  di- 
rection a  human  figure  of  enormous  dimensions.  A  gust  of  wind  near- 
ly blowing  off  his  hat  at  that  moment,  he  raised  his  hand  to  secure  it, 
and  the  colossal  figure  imitated  his  action.  Hane,  noticing  this,  at 
once  made  a  stooping  movement,  and  this  was  also  reproduced  by  the 


Fig.  35.— The  Spectre  of  the  Brocken. 


ANTHELIA.  131 

spectre.  He  then  called  another  person  to  him,  and  placing  themselves 
in  the  very  spot  where  the  apparition  was  first  seen,  the  pair  kept  their 
eyes  fixed  on  the  Achtermanrishohe,  but  saw  nothing.  After  a  short 
interval,  however,  two  colossal  figures  appeared,  which  repeated  the 
gestures  made  by  them,  and  then  disappeared. 

Some  few  years  ago,  in  the  summer  of  1862,  a  French  artist,  M. 
Stroobant,  witnessed  and  carefully  sketched  this  phenomenon,  which  is 
drawn  in  Fig.  35.  He  had  slept  at  the  inn  of  the  Brocken,  and  rising 
at  two  in  the  morning,  he  repaired  to  the  plateau  upon  the  summit  in 
the  company  of  a  guide.  They  reached  the  highest  point  just  as  the 
first  glimmer  of  the  rising  sun  enabled  them  to  distinguish  clearly  ob- 
jects at  a  great  distance.  To  use  M.  Stroobant's  own  words,  "My 
guide,  who  had  for  some  time  appeared  to  be  walking  in  search  of  some- 
thing, suddenly  led  me  to  an  elevation  whence  I  had  the  singular  priv- 
ilege of  contemplating  for  a  few  instants  the  magnificent  effect  of  mi- 
rage, which  is  termed  the  Spectre  of  the  Brocken.  The  appearance  is 
most  striking.  A  thick  mist,  which  seemed  to  emerge  from  the  clouds 
like  an  immense  curtain,  suddenly  rose  to  the  west  of  the  mountain,  a 
rainbow  was  formed,  then  certain  indistinct  shapes  were  delineated. 
First,  the  large  tower  of  the  inn  was  reproduced  upon  a  gigantic  scale ; 
after  that  we  saw  our  two  selves  in  a  more  vague  and  less  exact  shape, 
and  these  shadows  were  in  each  instance  surrounded  by  the  colors  of 
the  rainbow,  which  served  as  a  frame  to  this  fairy  picture.  Some  tour- 
ists who  were  staying  at  the  inn  had  seen  the  sun  rise  from  their  win- 
dows, but  no  one  had  witnessed  the  magnificent  spectacle  which-  had 
taken  place  on  the  other  side  of  the  mountain." 

Sometimes  these  spectres  are  surrounded  by  colored  concentric  arcs. 
Since  the  beginning  of  the  present  century,  treatises  on  meteorology 
designate,  under  the  name  of  the  Ulloa  circle,  the  pale  external  arch 
which  surrounds  the  phenomenon,  and  this  same  circle  has  sometimes 
been  called  the  "  white  rainbow."  But  it  is  not  formed  at  the  same  an- 
gular distance  as  the  rainbow,  and,  although  pale,  it  often  envelops  a 
series  of  interior  colored  arcs. 

Ulloa,  being  in  company  with  six  fellow  -  travelers  upon  the  Pam- 
bamarca  at  day-break  one  morning,  observed  that  the  summit  of  the 
mountain  was  entirely  covered  with  thick  clouds,  and  that  the  sun,  when 
it  rose,  dissipated  them,  leaving  only  in  their  stead  light  vapors,  which  it 
was  almost  impossible  to  distinguish.  Suddenly,  in  the  opposite  direc- 
tion to  where  the  sun  was  rising,  "each  of  the  travelers  beheld,  at  about 


132 


THE  ATMOSPHERE. 


seventy  feet  from  where  he  was  standing,  his  own  image  reflected  in  the 
air  as  in  a  mirror.  The  image  was  in  the  centre  of  three  rainbows  of 
different  colors,  and  surrounded  at  a  certain  distance  by  a  fourth  bow 
with  only  one  color.  The  inside  color  of  each  bow  was  carnation  or 
red,  the  next  shade  was  violet,  the  third  yellow,  the  fourth  straw  color, 
the  last  green.  All  these  bows  were  perpendicular  to  the  horizon; 
they  moved  in  the  direction  of,  and  followed,  the  image  of  the  person 
whom  they  enveloped  as  with  a  glory."  The  most  remarkable  point 
was  that,  although  the  seven  spectators  were  standing  in  a  group,  each 


Fig.  36.— The  Ulloa  circle. 

person  only  saw  the  phenomenon  in  regard  to  his  own  person,  and  was 
disposed  to  disbelieve  that  it  was  repeated  in  respect  to  his  companions. 
The  extent  of  the  bows  increased  continually  and  in  proportion  to  the 
height  of  the  sun ;  at  the  same  time  their  colors  faded  away,  the  spec- 
tres became  paler  and  more  indistinct,  and  finally  the  phenomenon  dis- 
appeared altogether.  At  the  first  appearance  the  shape  of  the  bows  was 
oval,  but  toward  the  end  they  became  quite  circular.  The  same  appa- 
rition was  observed  in  the  polar  regions  by  Scoresby,  and  described  by 
him.  He  states  that  the  phenomenon  appears  whenever  there  is  mist 
and  at  the  same  time  shining  sun.  In  the  polar  seas,  whenever  a  rather 


ANTHELIA.  ^33 

thick  mist  rises  over  the  ocean,  an  observer,  placed  on  the  mast,  sees 
one  or  several  circles  upon  the  mist. 

These  circles  are  concentric,  and  their  common  centre  is  in  the  straight 
line  joining  the  eye  of  the  observer  to  the  sun,  and  extended  from  the 
sun  toward  the  mist.  The  number  of  circles  varies  from  one  to  five; 
they  are  particularly  numerous  and  well  colored  when  the  sun  is  very 
brilliant  and  the  mist  thick  and  low.  On  July  23, 1821,  Scoresby  saw 
four  concentric  circles  around  his  head.  The  colors  of  the  first  and  of 
the  second  were  very  well  defined ;  those  of  the  third,  only  visible  at 
intervals,  were  very  faint,  and  the  fourth  only  showed  a  slight  greenish 
tint 

The  meteorologist  Kaemtz  has  often  observed  the  same  fact  in  the 
Alps.  Whenever  his  shadow  was  projected  upon  a  cloud,  his  head  ap- 
peared surrounded  by  a  luminous  aureola. 

To  what  action  of  light  is  this  phenomenon  due?  Bouguer  is  of 
opinion  that  it  must  be  attributed  to  the  passage  of  light  through  icy 
particles.  Such,  also,  is  the  opinion  of  De  Saussure,  Scoresby,  and  oth- 
er meteorologists. 

In  regard  to  the  mountains,  as  we  can  not  assure- ourselves  directly  of 
the  fact  by  entering  into  the  clouds,  we  are  reduced  to  conjecture.  The 
aerostat  traversing  the  clouds  completely,  and  passing  by  the  very  point 
where  the  apparition  is  seen,  affords  one  an  opportunity  of  ascertaining 
the  state  of  the  cloud.  This  observation  I  have  been  able  to  make,  and 
so  to  offer  an  explanation  of  the  phenomenon.* 

As  the  balloon  sails  on,  borne  forward  by  the  wind,  its  shadow  trav- 
els either  on  the  ground  or  on  the  clouds.  This  shadow  is,  as  a  rule, 
black,  like  all  others;  but  it  frequently  happens  that  it  appears  alone  on 
the  surface  of  the  ground,  and  thus  appears  luminous.  Examining  this 
shadow  by  the  aid  of  a  telescope,  I  have  noticed  that  it  is  often  com- 
posed of  a  dark  nucleus  and  a  penumbra  of  the  shape  of  an  aureola. 
This  aureola,  frequently  very  large  in  proportion  to  the  diameter  of  the 
central  nucleus,  eclipses  it  to  the  naked  eye,  so  that  the  whole  shadow 
appears  like  a  nebulous  circle  projected  in  yellow  upon  the  green 
ground  of  the  woods  and  meadows.  I  have  noticed,  too,  that  this  lu- 
minous shadow  is  generally  all  the  more  strongly  marked  in  proportion 
to  the  greater  humidity  of  the  surface  of  the  ground. 

Seen  upon  the  clouds,  this  shadow  sometimes  presents  a  curious  as- 

*  [The  explanation  of  the  phenomenon  offered  by  M.  Flammarion  (viz.,  that  it  is  due  to  dif- 
fraction) was  generally  recognized  long  previous  to  M.  Flammarion's  ascents. — ED.] 


Ig4:  THE  ATMOSPHERE. 

peek  I  have  often,  when  the  balloon  emerged  from  the  clouds  into  the 
clear  sky,  suddenly  perceived,  at  twenty  or  thirty  yards'  distance,  a  sec- 
ond balloon  distinctly  delineated,  and  apparently  of  a  grayish  color, 
against  the  white  ground  of  the  clouds.  This  phenomenon  manifests 
itself  at  the  moment  when  the  sun  re-appears.  The  smallest  details  of 
the  car  can  be  made  out  clearly,  and  our  gestures  are  strikingly  repro- 
duced by  the  shadow. 

On  April  15,  1868,  at  about  half- past  three  in  the  afternoon,  we 
emerged  from  a  stratum  of  clouds,  when  the  shadow  of  the  balloon  was 
seen  by  us,  surrounded  by  colored  concentric  circles,  of  which  the  car 
formed  the  centre.  It  was  very  plainly  visible  upon  a  yellowish  white 
ground.  A  first  circle  of  pale  blue  encompassed  this  ground  and  the 
car  in  a  kind  of  ring.  Around  this  ring  was  a  second  of  a  deeper  yel- 
low, then  a  grayish  red  zone,  and  lastly,  as  the  exterior  circumference, 
a  fourth  circle,  violet  in  hue,  and  imperceptibly  toning  down  into  the 
gray  tint  of  the  clouds.  The  slightest  details  were  clearly  discernible — 
net,  ropes,  and  instruments.  Every  one  of  our  gestures  was  instantane- 
ously reproduced  by  the  aerial  spectres.  The  anthelion  remained  upon 
the  clouds  sufficiently  distinct,  and  for  a  sufficiently  long  time,  to  permit 
of  my  taking  a  sketch  in  my  journal  and  studying  the  physical  condi- 
tion of  the  clouds  upon  which  it  was  produced.*  I  was  able  to  deter- 
mine directly  the  circumstances  of  its  production.  Indeed,  as  this  brill- 
iant phenomenon  occurred  in  the  midst  of  the  very  clouds  which  I  was 
traversing,  it  was  easy  for  me  to  ascertain  that  these  clouds  were  not 
formed  of  frozen  particles.  The  thermometer  marked  2°  above  zero. 
The  hygrometer  marked  a  maximum  of  humidity  experienced,  namely, 
77  at  3770  feet,  and  the  balloon  was  then  at  4600  feet,  where  the  hu- 
midity was  only  73.  It  is  therefore  certain  that  this  is  a  phenomenon 
of  the  diffraction  of  light  simply  produced  by  the  vesicles  of  the  mist. 

The  name  of  diffraction  is  given  to  all  the  modifications  which  the  lu- 
minous rays  undergo  when  they  come  in  contact  with  the  surface  of 
bodies.  Light,  under  these  circumstances,  is  subject  to  a  sort  of  devia- 
tion, at  the  same  time  becoming  decomposed,  whence  result  those  curi- 
ous appearances  in  the  shadows  of  objects  which  were  observed  for  the 
first  time  by  Grimaldi  and  Newton. 

The  most  interesting  phenomena  of  diffraction  are  those  presented  by 

*  A  colored  illustration  of  this  remarkable  phenomenon  is  given  in  the  Voyages  Aeriens, 
which  was  published  by  MM.  Glaisher,  De  Fonvielle,  and  G.  Tissandier,  in  conjunction  with 
myself,  part  2,  p.  292. 


ANTHELIA.  135 

gratings,  as.  are  technically  denominated  the  systems  of  linear  and  very 
narrow  openings  situated  parallel  to  one  another  and  at  very  small  in- 
tervals. A  system  of  this  kind  may  be  realized  by  tracing  with  a  dia- 
mond, for  instance,  on  a  pane  of  glass  equidistant  lines  very  close  to- 
gether. As  the  light  would  be  able  to  pass  in  the  interstices  between 
the  strokes,  whereas  it  would  be  stopped  in  the  points  corresponding  to 
those  where  the  glass  was  not  smooth,  there  is,  in  reality,  an  effect  pro- 
duced as  if  there  were  a  series  of  openings  very  near  to  each  other.  A 
hundred  strokes,  about  -^  of  an  inch  in  length,  may  thus  be  drawn 
without  difficulty.  The  light  is  then  decomposed  in  spectra,  each  over- 
lapping the  other.  It  is  a  phenomenon  of  this  kind  which  is  seen  when 
we  look  into  the  light  with  the  eye  half  closed ;  the  eyelashes,  in  this 
case,  acting  as  a  net-work  or  grating.  These  net-works  may  also  be 
produced  by  reflection,  and  it  is  to  this  circumstance  that  are  due  the 
brilliant  colors  observed  when  a  pencil  of  luminous  rays  is  reflected  on 
a  metallic  surface  regularly  striated. 

To  the  phenomena  of  gratings  must  be  attributed,  too,  the  colors,  oft- 
en so  brilliant,  to  be  seen  in  mother-of-pearl.  This  substance  is  of  a 
laminated  structure ;  so  much  so,  that  in  carving  it  the  different  folds 
are  often  cut  in  such  a  way  as  to  form  a  regular  net- work  upon  the  sur- 
face. It  is,  again,  to  a  phenomenon  of  this  sort  that  are  due  the  rain- 
bow hues  seen  in  the  feathers  of  certain  birds,  and  sometimes  in  spiders' 
webs.  The  latter,  although  very  fine,  are  not  simple,  for  they  are  com- 
posed of  a  large  number  of  pieces  joined  together  by  a  viscous  sub- 
stance, and  thus  constitute  a  kind  of  net-work. 

If  the  sun  is  near  the  horizon,  and  the  shadow  of  the  observer  falls 
upon  the  grass,  upon  a  field  of  corn,  or  other  surface  covered  with  dew, 
there  is  visible  an  aureola,  the  light  of  which  is  especially  bright  about 
the  head,  but  which  diminishes  from  below  the  middle  of  the  body. 
This  light  is  due  to  the  reflection  of  light  by  the  moist  stubble  .and  the 
drops  of  due.  It  is  brighter  about  the  head,  because  the  blades  that  are 
near  where  the  shadow  of  the  head  falls  expose  to  it  all  that  part  of 
them  which  is  lighted  up,  whereas  those  farther  off  expose  not  only  the 
part  which  is  lighted  up,  but  other  parts  which  are  not,  and  this  dimin- 
ishes the  brightness  in  proportion  as  their  distance  from  the  head  in- 
creases. The  phenomenon  is  seen  whenever  there  is  simultaneously 
mist  and  sun.  This  fact  is  easily  verified  upon  a  mountain.  As  soon 
as  the  shadow  of  the  mountaineer  is  projected  upon  a  mist,  his  head 
gives  rise  to  a  shadow  surrounded  by  a  luminous  aureola. 


136  THE  ATMOSPHERE. 

The  Illustrated  London  News  of  July  8,  1871,  illustrates  one  of  these 
apparitions,  "  The  Fog  Bow,  seen  from  the  Matterhorn,"  observed  by  E. 
Whymper  in  this  celebrated  region  of  the  Alps.  The  observation  was 
taken  just  after  the  catastrophe  of  July  14, 1865 ;  and  by  a  curious  co- 
incidence, two  immense  white  aerial  crosses  projected  into  the  interior 
of  the  external  arc.  These  two  crosses  were  no  doubt  formed  by  the 
intersection  of  circles,  the  remaining  parts  of  which  were  invisible.  The 
apparition  was  of  a  grand  and  solemn  character,  further  increased  by 
the  silence  of  the  fathomless  abyss  into  which  the  four  ill-fated  tourists 
had  just  been  precipitated. 

Other  optical  appearances  of  an  analogous  kind  are  manifested  under 
different  conditions.  Thus,  for  instance,  if  any  one,  turning  his  back  to 
the  sun,  looks  into  water,  he  will  perceive  the  shadow  of  his  head,  but 
always  very  much  deformed.  At  the  same  time  he  will  see  starting 
from  this  shadow  what  seem  to  be  luminous  -bodies,  which  dart  their 
rays  in  all  directions  with  inconceivable  rapidity,  and  to  a  great  dis- 
tance. These  luminous  appearances — these  aureola  rays — have,  in  ad- 
dition to  the  darting  movement,  a  rapid  rotatory  movement  around  the 
head. 


SALOS.  137 


CHAPTER  Y. 

• 

HALOS  :   PARHELIA — PARASELENES — CIRCLES  SURROUNDING  AND  TRAV- 
ERSING THE  SUN — CORONAS — COLUMNS — VARIOUS  PHENOMENA. 

THE  description  of  optical  phenomena  now  brings  us  to  one  of  the 
most  singular  and  complicated  effects  of  the  reflection  of  light  in  the  at- 
mosphere. Under  the  name  of  halo  (aXwg,  area)  is  designated  a  brill- 
iant circle  which,  under  certain  atmospheric  conditions,  surrounds  the 
sun  at  a  distance  of  22°  or  46° ;  while,  under  the  name  of  parhelia,  or 
mock  suns  (jrapa,  near,  and  rjXtoc,  sun),  are  designated  luminous  circu- 
lar spaces,  generally  of  a  red,  yellow,  or  greenish  color,  which  appear 
both  to  the  right  and  to  the  left  of  the  sun,  at  the  same  distance  (viz., 
about  22°),  bearing  a  sort  of  rough  resemblance  to  the  sun  itself.  The 
same  appearances  may  be  seen  about  the  moon  ;  and  it  is,  indeed,  easier 
to  observe  them,  as  the  diminished  brilliancy  of  the  moon's  light  ren- 
ders an  examination  of  the  area  around  it  less  difficult.  These  lumi- 
nous spaces  are  called  paraselenes  (irapa,  near,  and  <T£X^i/r},  moon),  or 
mock  moons.  The  two  cases  only  differ  as  to  the  intensity  of  the  lu- 
minary from  which  they  are  derived — a  difference  similar  to  that  which 
may  be  observed  between  ordinary  solar  and  lunar  rainbows. 

In  addition  to  the  halo  and  the  two  parhelia,  a  number  of  other  cir- 
cles, arches,  bands,  or  luminous  spots,  are  sometimes  seen  upon  the  sky. 
These  are  more  or  less  bright,  and  accompany  the  halo. 

It  is  well  known  that,  when  a  triangular  prism  of  glass  is  submitted 
to  the  action  of  the  sun's  rays,  part  of  the  light  falling  on  it  is  reflected 
from  the  surface  of  the  prism  as  upon  a  mirror,  and  another  part  pene- 
trates into  the  glass  and  leaves  it  in  a  direction  different  from  that  by 
which  it  entered,  producing  an  image  formed  of  different  colors.  It  is 
upon  this  fact  that  Mariotte  based  the  explanation  of  the  phenomenon 
which  we  are  about  to  consider.  The  origin  of  halos,  in  his  opinion,  is 
to  be  discovered  in  the  crystals  of  ice  in  the  shape  of  equilateral  trian- 
gular prisms  in  the  air.  These  prisms  may  be  situated  at  all  possible 
angles,  and  in  all  directions  in  the  atmosphere,  some  among  them  being 
in  such  positions  as  to  produce  the  absolute  minimum  of  deviation  of 
the  rays  of  light  which,  entering  by  one  of  the  three  lateral  surfaces  of 


Igg  THE  ATMOSPHERE. 

the  prisms,  traverse  one  of  the  other  two  on  their  way  out  of  it.  Mari- 
otte  has  shown  that,  at  an  angular  distance  from  the  sun  equal  to  that 
of  minimum  deviation,  which  is  22°,  a  brilliant  circle  must  be  formed, 
and  this  is  the  ordinary  halo.  If  from  some  cause  or  other  all  the 
prisms  become  vertical,  the  halo  is  replaced  by  two  parhelia.  The  tan- 
gent arcs  seen  near  the  ordinary  halo,  the  halo  with  a  radius  of  4.6°  and 
the  parhelion  circle,  have  been  explained  by  Young  upon  the  hypothe- 
sis that,  in  certain  cases,  the  prisms  may  be  situated  in  such  a  way  that 
their  axes  are  all  horizontal. 

Twenty  years  ago,  Bravais  devoted  to  the  analysis  of  these  phenom- 
ena a  work  which  will  be  useful  to  us  as  a  guide.  The  theory  of  these 
phenomena  is  somewhat  complex,  and  demands  a  certain  amount  of 
attention  in  order  to  be  intelligible.  Voltaire  confessed  that  he  was 
obliged  to  read  the  same  things  twice  over  in  order  to  comprehend 
them  thoroughly ;  and  perhaps  those  of  us  who  do  not  consider  our- 
selves more  acute  than  the  sage  of  Ferney  will  do  well  to  imitate  him 
in  this  instance. 

When  a  halo  appears  upon  the  sky,  light  cirri  clouds  (of  which  we 
shall  speak  presently)  are  generally  seen,  and  it  is  upon  them  that  the 
phenomenon  appears  to  be  delineated.  Sometimes,  too,  these  cirri  are 
collected  into  one  single  mass,  so  that  the  eye  can  not  seize  their  shapes : 
a  white  vapor  predominates  in  that  part  of  the  sky  near  to  the  sun ;  and 
the  blue  tint  of  the  atmosphere  is  replaced  by  a  kind  of  light  mist,  the 
brilliancy  of  which  is  sometimes  unbearable  to  the  eye.  But  these  light 
clouds  of  snow,  placed  high  in  the  air,  are  so  distant  that  it  is  difficult 
to  decide  upon  their  real  nature.  Hence  we  see  how  easily  the  mode  in 
which  the  phenomenon  is  produced  might  for  a  long  period  have  re- 
mained unknown ;  and  this  is  unquestionably  one  of  the  reasons  why 
halos  and  parhelia  were  in  early  ages  deemed  marvelous  phenomena, 
signs  of  celestial  ire,  presages  of  the  death  of  princes,  etc.,  etc. 

It  is  not  enough  for  the  clouds  of  the  higher  strata  of  the  atmosphere 
to  be  formed  of  snowy  particles  for  the  phenomenon  of  the  halo  to  be- 
come visible;  the  two  following  conditions  are  further  necessary.  The 
cloud  must  be  of  a  certain  degree  of  thickness ;  for,  if  too  thin,  the  halo 
would  not  occur;  if  too  dense,  the  light  would  be  intercepted.  The 
crystallization  of  the  water  must  also  proceed  slowly  and  not  be  disturb- 
ed by  wind,  as  with  a  rapid,  and  therefore  irregular,  crystallization  the 
points  lose  their  transparency,  the  angles  of  the  facets  their  consistency, 
and  the  surfaces  by  which  the  rays  enter  and  leave,  their  smoothness. 


HALOS.  139 

The  appearance  of  halos  is  less  rare  than  might  be  supposed.  It  is  cal- 
culated that  in  our  latitudes  the  number  of  days  on  which  this  phenom- 
enon occurs,  in  the  rudimentary  state  at  least,  are  fifty  a  year,  and  in  the 
north  of  Europe  many  more. 

The  most  simple  form  of  crystals  of  ice,  snow,  or  hoar-frost — viz., 
that  seen  in  the  earliest  process  of  crystallization — is  a  right  prism,  hav- 
ing for  its  section  a  regular  hexagon,  and  terminated  by  two  bases  per- 
pendicular to  the  lateral  surfaces,  which  are  rectangular. 

These  simple  forms  are,  however,  rarely  seen  in  a  fall  of  snow,  be- 
cause, before  reaching  the  ground,  lateral  crystallization,  due  to  the  con- 
densation of  vapor  in  the  lower  strata,  makes  an  addition  to  the  primi- 
tive nucleus. 

The  hexagonal  prism  gives  rise  to  all  the  spots  or  curves,  the  appear- 
ance of  which  has  been  placed  beyond  doubt  by  numerous  observa- 
tions. 

The  halo,  with  all  its  aspects,  is  explained  on  the  hypothesis  of  snow 
or  ice-crystals  falling  slowly  in  a  calm  atmosphere. 

It  is  therefore  due  simply  to  the  refraction  of  the  solar  rays  upon 
crystals  of  ice.  The  different  positions  of  the  prisms  of  ice  are  the  cause 
of  the  diversity  of  the  appearances.  The  situation  of  these  sharp-point- 
ed needles  of  ice  in  the  atmosphere  may  be  divided  into  three  classes: 
1st,  prisms  placed  at  any  angle ;  2d,  prisms  axes  of  which  are  vertical ; 
3d,  prisms  placed  horizontally. 

In  order  to  comprehend  the  production  of  the  phenomena,  let  us,  as 
in  explaining  the  rainbow,  take  the  first  case  and  examine  its  effects. 
If  a  prism  is  turned  round,  the  ray  which  emerges  from  it  is  seen  to 
make  a  variable  angle  with  that  which  enters  it.  But  there  is  a  certain 
position  in  which  the  entering  and  departing  rays  make  the  smallest 
angle  possible  with  each  other ;  the  prism  then  relative  to  the  incident 
ray  is  said  to  be  in  its  position  of  minimum  deviation.  Now,  in  this 
position,  the  prism  may  be  turned  a  little  one  way  or  a  little  the  other 
without  causing  any  perceptible  change  in  the  direction  of  the  refracted 
ray. 

If  a  prism  of  this  kind  turns  upon  its  own  axis  in  the  atmosphere, 
rays  are  incessantly  emanating  from  it,  which  reach  the  eye  and  disap- 
pear immediately  afterward;  but,  as  has  just  been  remarked,  it  is  clear 
that  the  ray  will  catch  the  eye  for  the  greatest  length  of  time  when  its 
deviation  is  a  minimum.  If  the  number  of  these  prisms  is  very  great, 
we  receive  at  the  same  time  the  rays  refracted  by  a  prism  at  the  mo- 


140 


THE  ATMOSPHERE. 


ment  at  which  the  others  disappear,  so  that  the  impression  upon  our 
eye  is  persistent,  although  the  rays  are  not  transmitted  to  it  by  the  same 
crystals.  A  solar  ray  enters  a  triangular  prism  by  the  surface  A  (see 
Fig.  37),  and  undergoes  a  deviation.  This  ray  is,  of  course,  decom- 
posed. Let  us  suppose  the  violet  portion,  after  emergence  from  the 
surface  B,  reaches  the  eye  of  the  spectator  placed  at  o.  Another  prism, 
c,  nearer  to  the  direction  o  a  of  the  sun,  will  send  red  rays  which  have 
deviated  the  least,  so  that  in  fact  the  cone  passing  through  A  will  be 
violet,  the  cone  passing  through  c  red,  and  the  intermediate  one  colored 
with  various  intermediate  colors  of  the  spectrum. 


Fig.  ST.— Theory  of  the  halo. 

Eefraction  of  the  solar  rays  will  thus  produce  all  round  the  sun,  and 
at  the  same  distance,  a  series  of  luminous  impressions.  The  deviation 
is  about  22°,  but  is  not  the  same  for  all  colors.  Calculation,  coinciding 
with  observation,  gives  21°  37'  for  the  red,  which  is  the  least  refrangi- 
ble color,  21°  48'  for  the  yellow,  21°  57'  for  the  green,  22°  10'  for  the 
blue,  and  20°  40'  for  the  violet.  This  circle  of  22°  radius  which  is  thus 
formed  around  the  sun  and  the  moon  is  the  ordinary  halo  which  is  seen 
most  frequently.  The  red  is  inside ;  then  we  have  orange,  yellow,  green ; 
but  these  colors  gradually  become  weaker,  because  they  are  influenced 
by  the  prisms,  which  are  not  in  the  position  of  minimum  deviation. 


HALOS.  141 

The  red  remains  most  visible.  The  sun,  however,  is  not,  as  we  have 
assumed,  a  mere  luminous  point,  but  each  part  of  its  disk  contributes 
to  the  production  of  this  phenomenon ;  and  this  circumstance  tends  to 
blend  still  further  the  various  colors,  which  are,  in  consequence,  never 
very  clearly  defined,  and  the  halo  generally  appears  as  a  bright  ring 
with  a  reddish  tint  on  the  inside,  2°  to  3°  in  width,  and  inclosing  a  cir- 
cular area  of  which  the  sun  occupies  the  centre. 

By  a  well-known  optical  effect,  a  spectator  not  previously  instructed 
upon  the  point  would  be  inclined  to  attribute  an  elliptic  shape  to  the 
halo,  considering  it  an  oval  with  a  longer  vertical  axis ;  but  this  illu- 
sion, which  also  takes  place  when  an  entire  rainbow  is  seen,  disappears 
before  angular  measurement.  From  a  similar  cause,  the  halo  appears 
to  get  smaller  as  the  sun  rises,  just  as  the  moon  loses,  at  a  certain  eleva- 
tion, the  gigantic  proportions  that  its  disk  presented  soon  after  rising. 
In  addition  to  the  halo  of  22°  radius,  a  second  is  also  frequently  seen, 
the  diameter  of  which  is  about  twice  as  large  as  that  of  the  preceding 
one. 

The  latter  is  produced  by  the  refraction  of  light  across  the  dihedral 
angles  of  90°  that  the  sides  of  the  prisms  make  with  the  bases,  just  as 
the  angles  of  60°  produce  the  ordinary  halo.  Like  the  latter,  it  is  com- 
posed of  a  succession  of  rings,  the  first  of  which  (viz.,  the  one  nearest  to 
the  sun)  is  -red.  But,  by  a  superposition  of  colors  similar  to  that  which 
occurs  in  the  halo  of  22°,  there  is  scarcely  discernible  more  than  a  ring, 
reddish  upon  its  inside  and  yellowish  in  the  middle,  whereas  the  exter- 
nal part  seems  of  a  whitish  hue,  and  gradually  becomes  lost  in  the  gen- 
eral light  of  the  atmosphere.  The  total  width  of  this  halo  is  rather 
large,  embracing  about  the  8°  between  45°  and  48°  distance  from  the 
sun,  the  white  light  that  borders  it  included. 

These  two  circles  are,  therefore,  formed  by  the  reflection  of  light  upon 
the  prisms  of  ice  placed  at  all  angles  in  the  air.  Let  us  now  consider 
what  effects  may  be  produced  by  prisms  placed  vertically.  When  the 
light  is  reflected  across  the  dihedral  angles  of  60°,  which  the  six  sides 
of  the  prisms  of  ice  falling  vertically  form  between  them,  there  are  two 
parhelia  produced,  one  to  the  right,  the  other  to  the  left  of  the  sun,  and 
both  situated  at  the  same  height  as  the  latter.  To  rightly  understand 
the  reason  of  this  phenomenon,  the  principle  must  first  be  enunciated 
that  the  light  given  by  a  group  of  prisms,  all  of  which  have  their  axes 
vertical,  but  which  are  situated  in  every  conceivable  position  as  to  the 
direction  of  their  sides,  is  similar  to  that  which  would  be  transmitted 


-j^2  THE  ATMOSPHERE. 

by  a  single  prism  turning  rapidly  on  its  own  axis.  It  follows,  in 
fact,  that  the  prism,  in  the  movement  indicated  above,  passes  in  suc- 
cession through  all  the  positions  compatible  with  the  verticality  of  its 

axis. 

When  the  sun  is  on  the  horizon,  the  distance  at  which  these  appear- 
ances are  formed  is  exactly  the  angle  of  minimum  deviation,  or,  in  oth- 
er words,  the  radius  of  the  halo.  If  the  halo  and  the  parhelia  are  seen 
together,  the  latter  appear  to  be  situated  just  upon  the  circumference  of 
the  prism,  and  occupy  in  height  a  distance  equal  to  the  diameter  of  the 
sun.  The  various  tints  are  clearer  than  in  the  halo ;  the  yellow  is  very 
distinct,  and  so  is  the  green,  but  the  blue  is  pale,  and  scarcely  visible; 
while  the  violet,  overlapped  by  the  other  colors,  is  too  indistinct  to  be 


Pig.  38,-Halo  seen  ID  Norway. 

seen.  The  phenomenon  is  completed  by  a  tail  of  white  light,  some- 
times very  indistinct,  but  occasionally  attaining  a  length  of  from  10°  to 
20°  in  the  opposite  direction  to  the  sun,  and  parallel  to  the  horizon. 
This  light  is  due  to  those  prisms,  the  positions  of  which  are  somewhat 
out  of  the  line  that  corresponds  to  the  minimum  deviation. 

When  the  sun  rises  about  the  horizon,  the  luminous  rays  traverse  the 
prisms,  moving  in  oblique  planes,  and  the  smallest  of  the  deviations 
produced  during  the  rotation  is  greater  than  the  absolute  corresponding 
minimum,  when  the  sun  is  at  the  horizon.  This  shows  that  the  parhe- 
lia must  emerge  slowly  from  the  circumference  of  the  halo,  in  propor- 
tion as  the  latter  rises  in  height;  but  on  the  other  hand,  as  the  halo  is 
nearly  2°  in  width  (including  the  white  light  that  borders  it),  the  par- 


HAL  OS.  143 

helia  only  become  completely  separated  from  it  when  the  sun  is  at  an 
elevation  of  20°  or  30°. 

Optical  considerations  show  that  the  formation  of  parhelia  becomes 
impossible  when  the  sun  has  reached  an  elevation  of  60°. 

Parhelia  are  sometimes  very  brilliant,  and  their  brightness  may  then 
be  in  a  certain  measure  compared  to  that  of  the  sun  itself,  in  which  case 
it  is  quite  conceivable  that  each  parhelion  may  become  in  its  turn  the 
origin  of  two  others,  which  are  then  the  parhelia  of  parhelia,  or  seconda- 
ry parhelia.  The  effect  caused  by  the  refraction  of  light  across  angles 
of  90°,  which  produce  the  large  halo,  is  still  more  remarkable.  The  so- 
lar rays  enter  obliquely  at  the  upper  base  of  the  prisms,  and,  passing 
through  it,  emerge  by  one  of  the  vertical  surfaces. 

If  we  imagine,  as  we  have  already  done  in  the  case  of  the  parhelia, 
that  the  prism  on  the  upper  base  of  which  the  rays  are  falling,  turns 
rapidly  upon  its  own  axis,  it  may  be  proved  by  optics  that  the  light 
emerging  from  it  will  be  scattered  in  the  form  of  a  bright  curve  with  its 
axis  vertical,  whence  it  is  easy  to  conclude  that  the  corresponding  optic- 
al appearance  upon  the  celestial  sphere  will  be  a  luminous  arc  parallel 
to  the  horizon  and  situated  at  a  great  distance  above  the  sun. 

The  arc  thus  produced,  which  may  be  termed  the  upper  tangent  arc  of 
the  halo  0/"46°,  or,  more  briefly,  the  circumzenithal  arc,  deserves  special 
notice,  for  it  is  unquestionably  the  most  remarkable  of  all  the  appear- 
ances which  may  accompany  the  halo.  The  brightness  of  the  tints,  the 
distinctness  of  the  colors,  the  precision  with  which  -its  edges,  as  well  as 
its  extreme  limits,  are  shown  upon  the  sky,  give  it  the  characteristic  of 
a  real  rainbow.  Of  the  respective  rings  composing  it,  the  red  is  nearest 
to  the  sun,  the  violet  fringes  the  concave  part  of  the  arc,  and  is  on  the 
opposite  side ;  the  width  of  the  various  rings  is  about  the  same  as  in  the 
rainbow,  though  rather  less,  owing  to  the  illusion  caused  by  the  prox- 
imity of  the  zenith.  When  the  halo  of  46°  is  visible,  the  circumzenithal 
arc  generally  appears  to  touch  it  at  its  highest  point,  the  red  of  the  arc 
being  then  in  contact  with  the  red  of  the  halo,  the  orange  with  the  or- 
ange, and  so  on  with  the  other  colors;  but  very  often  the  circumze- 
nithal arc  is  seen  without  the  halo  of  46°,  just  as  the  parhelia  may  ap- 
pear without  the  halo  of  22°,  although  they  owe  their  existence  to  the 
same  kind  of  dihedral  angles. 

From  the  observations  that  have  been  made  of  this  arc,  it  appears 
that  it  never  is  to  be  seen  when  the  height  of  the  sun  is  less  than  12°  or 
more  than  31°. 


144  THE  ATMOSPHERE. 

It  follows  also  from  optical  consideration  that  prisms,  falling  and 
turning  upon  their  sides,  can  reflect  the  sun,  forming  upon  the  celestial 
sphere  a  luminous  horizontal  band,  extending  right  round  the  horizon 
and  passing  through  the  exact  centre  of  the  sun.  As  reflection  does 
not  separate  the  colors  which  compose  white  light,  this  circle  will  ap- 
pear to  be  quite  white,  and  its  apparent  width  will  be  equal  to  the  di- 
ameter of  the  sun.  Such  is  the  origin  of  the  white  circle  called  the  par- 
heliacal  ring.  It  is  upon  its  circumference  that  the  ordinary  parhelia 
always  appear,  as  also  the  secondary  parhelia  situated  at  about  45°  from 
the  sun;  hence  the  name. 

Sometimes  the  solar  rays  experience  two  successive  reflections  upon 
the  vertical  surfaces  of  one  of  the  prisms.  There  is  then  visible,  at 
120°  from  the  sun,  a  white  image  more  or  less  diifuse,  which  has  re- 
ceived the  name  of  paranthelion.  The  horizontal  bases  of  the  ice-crys- 
tals reflect  also  the  solar  light,  but  in  an  upward  direction,  which  pre- 
vents the  spectator  from  perceiving  it,  unless  he  be  upon  the  summit  of 
a  steep  mountain,  or  in  the  car  of  a  balloon,  above  the  cloud  containing 
the  icy  particles.  It  will  be  readily  admitted  that  these  conditions  can 
be  rarely  fulfilled ;  but  MM.  Barral  and  Bixio  were,  fortunately,  able  to 
realize  them  on  July  27th,  1850.  The  image  of  the  sun  thus  reflected 
appeared  almost  as  luminous  as  the  sun  itself.  Bravais  suggested  for 
this  phenomenon,  at  once  so  remarkable  and  so  rare,  the  name  of  pseu- 
dohelion. 

Finally,  the  prisms  of  ice  which  are  horizontal  in  the  atmosphere  give 
rise,  by  reflections  and  refractions  analogous  to  the  above,  to  tangent 
arcs  which  often  appear  on  each  side  of  the  halo. 

The  most  complete  halo  that  has  yet  been  seen  is  that  which  Lowitz 
observed  at  St.  Petersburg,  on  June  29, 1790,  from  7  hours  30  minutes 
A.M.  to  12  hours  30  minutes  P.M.  Since  that  time  there  have,  of  course, 
been  a  great  number  of  halos  observed ;  but  this  is,  perhaps,  the  most 
complete  that  has  been  recorded.  MM.  Bravais  and  Martins  observed 
one  at  Piteo,  in  Sweden,  on  October  4,  1839,  which  was  also  very  re- 
markable, but  less  complete  than  that  seen  by  Lowitz. 

The  examination  which  we  have  made  of  the  general  phenomenon 
of  halos  leads  us  to  speak  of  other  optical  effects,  the  explanation  of 
which  is  more  or  less  akin  to  that  of  the  above. 

The  columns  of  white  light,  the  crosses,  and  the  different  luminous 
aspects  sometimes  visible  at  sunrise  and  sunset,  are  due  to  the  reflection 
of  light  upon  the  surfaces  of  crystals  of  ice  situated  high  in  the  atmos- 


HAL08.  145 

phere.  It  is  well  known  that  if  we  look  at  the  reflection  of  any  light 
(such  as  the  sun,  the  moon,  or  a  street-lamp)  on  the  surface  of  rather 
troubled  water,  the  reflection  extends  vertically ;  the  motion  of  the  wa- 
ter gives  rise  to  a  multitude  of  small  surface-planes  which  are  oscillating 
unceasingly  about  the  horizontal,  in  all  possible  directions.  This  is  the 
exact  reproduction  of  what  is  going  on  in  the  region  of  the  ice-cloud ; 
the  small  coruscating  bases  of  the  prisms,  to  which  I  have  attributed 
above  the  reflection  of  the  sun  as  seen  from  a  balloon,  are  perpetually 
shifting  their  position.  The  reflection  produced  will  therefore  also  be 
very  elongated,  and  its  upper  part  may,  at  sunrise  or  sunset,  rise  several 
degrees  above  the  horizon. 

Such  is  the  origin  of  those  columns  of  white  light  which  are  some- 
times seen  to  form  at  the  moment  of  sunset,  and  to  increase  in  size  as  the 
sun  gradually  sinks  lower.  It  is  scarcely  necessary  to  add  that,  when 
the  sun  has  descended  below  the  horizon,  the  reflection  of  the  light 
takes  place  at  the  lower  and  not  at  the  upper  surfaces  of  the  prisms. 

Previous  to  sunset,  on  April  22,  1847,  four  luminous  columns,  each 
about  15°  in  extent,  were  seen  from  Paris,  presenting  the  appearance  of 
a  cross  with  the  sun  in  the  centre.  After  sunset  one  of  these  four  col- 
umns (the  uppermost  of  the  four,  of  course)  still  remained  visible  for 
some  little  time. 

When  the  sun  is  near  the  horizon,  part  of  a  vertical  circle  may  rise 
above  that  luminary  in  the  shape  of  a  column.  On  June  8, 1824,  ap- 
pearances of  this  kind  were  seen  in  several  parts  of  Germany.  At 
Dohna,  near  Dresden,  at  eight  in  the  evening,  just  as  the  sun  was  about 
to  disappear  behind  the  mountains,  Lohrmann  perceived  a  luminous 
band,  perpendicular  to  the  crepuscular  arc,  and  similar  to  the  tail  of  a 
comet.  This  column  was  30°  high  and  1°  in  width.  It  is  more  unusual 
to  see  a  band  below  the  sun  or  the  moon,  and  more  unusual  still  to  see 
also  a  horizontal  arc  passing  the  sun  in  such  a  way  that  it  is  situated  in 
the  middle  of  a  cross.  Roth  saw  very  distinctly  a  phenomenon  of  this 
kind  at  Cassel,  on  January  2, 1586.  Before  the  sun^  appeared,  a  lumi- 
nous vertical  column,  with  a  diameter  equal  to  that  of  the  sun,  was  visi- 
ble at  the  spot  where  the  sun  was  about  to  rise,  resembling  a  brilliant 
flame,  except  that  its  brightness  was  of  uniform  intensity  throughout. 
Soon  after  there  appeared  a  reflection  of  the  sun,  so  brilliant  that  it  was 
taken  for  the  sun  itself;  and  this  parhelion  had  scarcely  risen  above  the 
horizon  when  the  sun  rose  immediately  under  it,  followed  by  a  column 
resembling  that  which  had  appeared  above  it. 

10 


146  THE  ATMOSPHERE. 

This  latter,  with  its  three  suns,  remained  continuously  vertical.  The 
three  suns  were  each  exactly  similar  in  appearance,  but  the  true  sun 
was  the  most  brilliant.  The  phenomenon  lasted  about  an  hour. 

If  the  sun,  instead  of  being  on  the  horizon,  is  some  few  degrees  above 
it,  the  luminous  column  which  rises  from  the  pseudohelion  then  situ- 
ated below  the  horizon,  and  consequently  invisible,  may  reach  to  the 
centre  of  the  sun,  but  can  not  extend  perceptibly  beyond  it.  We  then 
have  the  appearance  of  a  luminous  ascending  column,  which  seems  to 
support  the  solar  disk.  Instances  of  this  are  afforded  by  the  observa- 
tions taken  by  Parry  at  Melville  Island  on  March  8,  1820 ;  by  Sturm 
on  December  9, 1689 ;  and  by  many  others. 

The  vertical  gleams  which,  passing  through  the  centre  of  the  sun,  ex- 
tend symmetrically  above  and  below  it,  without  having  their  base  at 
the  horizon,  and  which  accompany  the  sun  in  his  apparent  course  from 
east  to  west,  seem  due  to  the  same  cause.  It  is  easy  to  see  that  they  are 
caused  by  the  rays  twice  reflected  upon  the  horizontal  bases  of  the  ver- 
tical prisms,  or,  at  all  events,  by  some  even  number  of  successive  reflec- 
tipns.  They  are  never  seen  but  at  heights  less  than  25° ;  and  are  far 
more  frequently  seen  about  the  moon  than  about  the  sun — a  fact  which 
is  no  doubt  due  to  the  greater  brightness  of  the  latter,  which  thus  eclipses 
all  the  gleams  near  to  it.  The  reverse  is  the  case  with  the  columns 
which  are  seen  at  sunset,  because  the  sun  then  being  below  the  horizon, 
the  phenomenon  is  projected  upon  a  partially  lighted  ground,  and  may 
thus  be  seen  in  all  its  brilliancy. 

The  combination  of  the  parheliacal  circle  with  the  vertical  column 
passing  through  the  centre  of  the  sun,  produces  the  phenomenon  of  the 
solar  or  lunar  crosses  which  are  often  seen  when  the  halo  of  22°  is  not 
visible.  Sometimes  the  arms  of  the  cross  may  be  nearly  equal  in  length, 
and  sometimes  the  horizontal  are  larger  than  the  vertical. 

The  vertical  columns,  and  the  lunar  and  solar  crosses,  are  mostly  seen 
in  northern  countries  during  the  long  winters  which  envelop  those  re- 
gions in  snow  and  ice. 

To  these  optical  effects  must  be  added,  finally,  the  coronas  (see  Fig. 
39)  which  appear  around  the  sun  and  the  moon  when  the  air  is  not 
clear,  and  when  small  drops  of  vesicular  vapor,  or  light  clouds,  are 
passing  before  their  bodies. 

These  colored  rings,  which  are  frequently  seen  round  the  moon,  owe 
their  origin  not  to  refraction,  but  to  diffraction  ;  they  have  the  red  out- 
side, and  the  violet  inside,  like  the  primary  rainbow,  and  their  colors 


HALOS.  147 

are  the  converse  of  those  of  the  two  halos  concentric  with  the  sun  and 
moon.  The  diameters  of  coronas  of  the  same  color  are  in  the  propor- 
tion of  the  natural  numbers,  1,  2,  3,  4,  etc.,  but  the  diameter  of  the  first 
ring  seems  enlarged.  This  diameter,  varying  from  1°  to  4°,  depends 
upon  that  of  the  vesicles  of  water  interposed  between  the  luminary  and 
the  observer.  Generally,  the  color  of  it  is  blue  mixed  with  white  for  a 
certain  distance  round  the  luminary ;  then  follows  a  red  circle,  and  then 
other  colored  circles,  as  in  Newton's  rings.  For  the  phenomenon  to 


Fig.  39.— Corona  formed  around  the  moon  by  diffraction. 

take  place  there  must  be  a  certain  number  of  globules  of  the  same  char- 
acter, and,  indeed,  a  far  greater  number  of  this  diameter  than  of  any 
other.  If  the  diameters  of  the  spherules  of  cloud  were  all  different,  the 
corona  would  not  be  produced.  An  exactly  similar  effect  is  observable 
when  a  luminous  object  is  examined  through  a  piece  of  glass  that  has 
been  sprinkled  with  lycopodium  powder,  or,  in  a  less  marked  degree,  if 
the  glass  has  merely  been  breathed  upon  before  use. 

To  these  different  effects,  due  to  the  refraction  and  reflection  of  light 
in  the  atmospheric  strata,  must  also  be  added  the  deformation  of  the 


148  THE  ATMOSPHERE. 

sun  at  the  horizon,  which  occasionally  gives  rise  to  most  singular  ap- 
pearances, in  consequence  of  the  want  of  homogeneousness  in  the  lower 
strata,  and  the  curious  action  of  atmospheric  refraction. 

With  the  progress  of  astronomy  and  physics,  the  decadence  of  astrol- 
ogy, and  the  expansion  of  inquiry,  these  optical  phenomena  lose  their 
supernatural  attributes.  For  the  last  century  they  have  undergone  a 
calm  and  impartial  study  and  analysis ;  while  we  see  in  this  chapter  that 
they  may  be  explained  upon  theory,  and  savans  merely  recognize  them 
as  so  many  physical  facts  belonging  to  the  vast  domain  of  meteorology. 
The  historian  Josephus  relates  that  at  the  beginning  of  the  siege  of  Je- 
rusalem by  the  Komans,  A.D.  70,  the  Jews  foresaw  their  disaster  "  in  ar- 
mies marching  upon  red  clouds."  Nearly  analogous  apparitions  were 
visible  at  the  commencement  of  the  siege  of  Paris  in  September,  1870, 
to  say  nothing  of  the  aurora  borealis  on  the  24th  of  October;  but  we 
now  know  that  the  physical  effects  are  purely  natural,  and  are  produced 
merely  by  the  action  of  light  in  the  atmosphere. 


I 


THE  MIRAGE. 


CHAPTER  VI. 
THE  MIRAGE. 

NOT  only  does  the  atmosphere  produce  remarkable  phenomena  in 
the  aerial  heights,  but  it  gives  play  to  its  fancy  even  in  the  lower  re- 
gions where  we  move,  and  the  very  surface  of  the  ground  and  of  the 
water  is  occasionally  the  field  of  strange  metamorphoses  due  to  the  rays 
of  light  in  the  air. 

Under  the  name  of  mirage  we  designate  those  optical  apparitions 
caused  by  a  peculiar  state  of  the  densities  of  the  atmospheric  strata — a 
state  which  produces  variations  in  the  ordinary  refractions  which  we 
considered  in  the  previous  chapter. 

In  consequence  of  these  variations  distant  objects  appear  either  de- 
formed, transported  to  a  certain  distance,  or  inverted  and  reflected,  ac- 
cording to  the  deviation  which  the  abnormal  density  of  the  air  causes 
in  the  luminous  rays. 

The  mirage  is  no  new  phenomenon.  In  Diodorus  Siculus  we  read : 
"An  extraordinary  phenomenon  occurs  in  Africa  at  certain  periods, 
especially  in  calm  weather ;  the  air  becomes  filled  with  images  of  all 
sorts  of  animals,  some  motionless,  others  floating  in  the  air:  now  they 
seem  running  away,  now  pursuing ;  they  are  all  of  enormous  propor- 
tions, and  this  spectacle  fills  with  terror  and  awe  those  who  are  not  ac- 
customed to  it.  When  these  figures  overtake  the  traveler  whom  they 
seem  to  be  pursuing,  they  surround  him  with  a  cold  and  shivering  feel- 
ing. Strangers  not  used  to  this  extraordinary  phenomenon  are  seized 
with  fear ;  but  the  inhabitants,  who  are  in  the  habit  of  seeing  it,  take 
no  particular  notice  of  it. 

"  Certain  physical  philosophers  attempt  to  explain  the  true  causes  of 
this  phenomenon,  which  seems  extraordinary  and  fabulous.  They  say 
that  there  is  no  wind,  or  scarcely  any,  in  this  country.  The  masses  of 
condensed  air  produce  m  Libya  what  the  clouds  sometimes  produce 
with-iis-QiL rainy  days^  viz.,  images  of  all  shapes  rising  on  every  side  in 
the  air.J  These  strata  of  air,  suspended  by  light  breezes,  become  mixed 
with  other  strata,  executing  at  the  same  time  very  rapid  oscillatory 
movements;  and  when  calm  again  sets  in  they  descend  toward  the 


150  THE  ATMOSPHERE. 

ground  by  their  own  weight,  preserving  the  shapes  that  they  had  ac- 
cidentally assumed.  If  no  cause  occurs  to  disperse  them,  they  spon- 
taneously attach  themselves  to  the  first  animals  which  come  near. 
Their  movements  do  not  appear  to  be  the  effect  of  volition,  for  it  is  im- 
possible for  an  inanimate  being  to  progress  or  go  backward.  But  it  is 
the  animated  beings  who,  unwittingly,  produce  these  voluntary  move- 
ments, for,  as  they  advance,  they  cause  a  violent  recoil  in  the  images 
which  seem  to  fly  before  them.  Similarly,  those  which  recoil  seem,  by 
producing  a  void  and  a  relaxation  in  the  strata  of  the  air,  to  be  pursued 
by  the  aerial  spectres!]  The  persons  running  away  are  probably  struck, 
when  they  stop  or  return  to  their  former  position,  by  the  matter  of  these 
figures,  which  break  against  their  bodies  and  produce,  at  the  moment 
of  the  shock,  the  chilly  sensation." 

We  see  that,  before  the  epoch  of  Diodorus,  the  mirage  had  been  ob- 
served ;  the  philosophers  of  the  period  were  nevertheless  far  from  being 
in  possession  of  the  true  scientific  explanation,  although  it  was  then  at- 
tributed to  a  change  of  density  in  the  aerial  strata. 

This  same  phenomenon  (of  which  Quintus  Curtius  has  also  spoken) 
has  long  been  remarked  by  the  Arabs,  and  it  is  often  discussed  in  the 
treatises  of  Oriental  writers.  Among  other  instances  may  be  cited  the 
Koran,  which  says  that  "the  works  of  the  incredulous  are  like  the  mi- 
rage (swab)  of  the  plain;  the  thirsty  man  takes  it  for  water  until  he 
draws  nigh  to  it,  and  then  he  discovers  that  it  is  nothing." 

In  about  the  middle  of  the  seventeenth  century  the  mirage  began  to 
attract  the  special  attention  of  physicists.  The  discovery  of  telescopes 
rendered  possible  a  great  number  of  observations,  which  were  beyond 
the  power  of  the  naked  eye ;  and  the  knowledge  of  the  laws  of  the  re- 
fraction of  light,  and  of  the  variations  in  the  density  of  the  air  caused 
by  changes  in  its  temperature,  prepared  the  way  for  the  theoretical  ex- 
planation of  these  singular  apparitions.  It  is  not  till  1783  thafc-we-£«fl 
Ifhe  first  really  scientific  work  treating  of  the  mirage.'  This  was  from 
the  pen  of  Professor  Busch,  who  observed  its  effects  on  the  Elbe,  near 
Hamburg,  and  on  the  coasts  of  the  Northern  and  Baltic  seas.  He  often 
made  use  of  a  telescope,  and  this  method  of  observation  disclosed  to  him 
many  details  hitherto  unknown.  He  saw  upon  several  occasions  a  mir- 
ror of  the  waters  and  mock  bank,  beneath  which  figures  upside  down 
seemed  to  be  delineated;  he  saw  ships  suspended  in  the  air,  and  bearing 
beneath  their  keels  the  reversed  image  of  their  mnsts  and  sails.  On 
the  5th  October,  1779,  he  saw,  at  the  distance  of  two  German  miles  from 


THE  MISAOK  151 

Bremen,  the  ordinary  image  of  that  town  and  a  second  image,  very  dis- 
tinct but  upside  down ;  between  him  and  the  town  there  was  a  large 
and  verdant  common.  The  principal  circumstances  of  the  phenomenon 
are  clearly  indicated  in  his  work,  without,  however,  the  theoretical  ex- 
planation of  them. 

It  was  during  Bonaparte's  expedition  to  Egypt  that  the  true  expla- 
nation of  the  phenomenon  was  first  given. 

The  soil  of  Lower  Egypt  forms  a  vast  and  perfectly  horizontal  plain, 
the  uniformity  of  which  is  only  broken  by  gentle  eminences  upon  which 
are  built  the  villages  that  are  thus  protected  from  the  overflowings  of 
the  Nile.  At  morning  and  evening  there  is  no  change  in  the  aspect  of 
the  country;  but  when  the  sun  has  heated  the  surface  of  the  soil,  it 
seems,  at  a  certain  distance  off,  to  be  inundated;  the  villages  look  like 
islands  in  the  middle  of  an  immense  lake,  and  below  each  village  is  to 
be  seen  its  inverted  reflection.  To  complete  the  illusion,  the  ground 
vanishes,  and  the  vault  of  the  firmament  is  apparently  reflected  in  still 
water.  It  is  easy  to  understand  the  cruel  disappointment  of  the  French 
army.  Exhausted  by  fatigue,  with  a  devouring  thirst  under  the  burn- 
ing sky,  the  men  fancied  they  had  reached  a  great  pool  of  still  water  in 
which  they  saw  reflected  the  shadow  of  the  villages  and  the  palm-trees; 
but  as  they  gradually  approached,  the  limits  of  this  seeming  inundation 
retreated ;  the  imaginary  lake,  that  appeared  to  surround  the  village, 
drew  back,  and  finally  melted  away  altogether,  the  same  illusion  being 
repeated  in  the  case  of  the  next  village.  The  savans  attached  to  the 
expedition  who  witnessed  this  phenomenon  were  not  less  surprised  than 
the  rest  of  the  army ;  but  Monge  succeeded  in  giving  the  explanation 
of  it. 

The  theory  of  the  mirage,  in  order  to  be  perfectly  understood,  de- 
mands very  special  attention.  The  phenomenon  occurs  when  the  lu- 
minous rays,  through  whose  agency  we  see  objects,  are  made  (before^ 
they  reach  our  eye)  to  undergo  a  deviation  caused  by  differences  of 
density  in  the  strata  of  air  they  pass  through.  We  have  seen  that 
when  a  luminous  ray  penetrates  from  a  less  dense  into  a  more  dense 
medium,  it  undergoes  a  deviation  which  bends  it  nearer  to  the  line  per- 
pendicular to  the  boundaries  of  the  two  surfaces;  and  when  it  passes 
from  a  more  dense  to  a  less  dense  medium,  it  suffers  a  deviation  bend- 
ing it  from  the  perpendicular. 

Further,  the  angle  of  refraction  is  greater  than  the  angle  of  incidence, 
and  at  a  given  moment  a  certain  ray  will,  after  refraction,  make  an  an- 


152  THE  ATMOSPHERE. 

gle  of  90°  with  the  perpendicular  to  the  surface.  This  is  called  the 
critical  angle. 

Beyond  this  angle  the  rays  are  reflected,  and  do  not  enter  the  me- 
dium at  all ;  this  is  known  in  physics  under  the  name  of  total  reflection. 

An  illustration  of  this  fact  may  be  obtained  by  filling  a  glass  with 
water  and  holding  it  so  as  to  see  the  surface  of  the  liquid  from  under- 
neath ;  this  surface  acts  like  a  mirror,  and  appears  very  bright.  A 
spoon  dipped  into  it  is  reflected.  Another  instance :  a  prism  of  glass 
properly  placed  at  the  opening  of  a  dark  room  is  capable  of  intercepting 
entirely  the  passage  of  light  by  this  very  fact  of  total  reflection.  In 
fact,  when  a  luminous  ray  tends  to  emerge  from  a  more  reflecting  me- 
dium into  one  that  is  less  so,  at  an  angle  greater  than  the  critical  angle, 
the  ray  is  entirely  reflected. 

This  being  taken  for  granted,  we  may  now  affirm  that  the  mirage  is 
a  phenomenon  of  total  reflection. 

By  the  action  of  the  solar  rays,  when  the  atmosphere  is  calm,  the 
strata  of  air  which  are  in  contact  with  the  soil  become  very  much  heat- 
ed, and  it  may  happen  that  for  a  short  distance  up  their  density  may  in- 
crease as  they  are  farther  from  the  ground.  This  is  a  purely  accidental 
fact,  which  depends  upon  various  circumstances  peculiar  to  the  place 
where  it  occurs ;  it  does  not  extend  very  far,  and  consequently  in  nowise 
affects  the  general  law  of  the  decrease  of  density  in  proportion  to  the 


Pig.  40.— Explanation  of  the  ordinary  mirage. 

elevation.  In  the  event  of  these  physical  conditions  happening,  the  fol- 
lowing may  be  the  result:  a  luminous  ray,  starting  from  the  point  M 
(see  Fig.  40),  is  successively  refracted  in  a'  <f ,  as  it  is  bent  from  the  nor- 
mal ;  at  a  given  moment  the  direction  will  coincide  with  that  of  the 


THE  MIRAGE.  153 

stratum  of  air  A,  and  this  latter  will  serve  as  a  mirror :  the  ray  will  fol- 
low, therefore,  in  an  opposite  direction  a  path,  A  d'  a',  similar  to  that 
which  it  has  already  taken,  and  will  reach  the  eye  of  the  spectator,  who 
will  see  in  the  lower  direction,  o  M,  the  reflection  of  the  palm-tree  M,  at 
the  same  time  that  he  will  see  the  object  directly.  It  is,  therefore,  the 
stratum  of  air  which,  at  a  given  moment,  becomes  a  mirror,  and  con- 
sequently acts  in  the  same  way  as  a  piece  of  reflecting  water,  which 
gives  rise  to  the  phenomenon.  Such  is  the  ordinary  or  inferior  mirage. 

This  lower  and  reflected  deviation  of  the  luminous  rays  does  not  al- 
ways attract  attention  so  much  as  might  be  fancied.  Many  people  will 
pass  by  it  without  remarking  it,  and,  even  when  their  attention  is  called 
to  the  fact,  will  declare  that  they  perceive  nothing  extraordinary  or 
worthy  of  notice.  To  clearly  discern  the  mirage,  a  person  must  not 
only  possess  long  and  very  accurate  eyesight,  but  must  also  know  how 
to  observe  details,  and  be  accustomed  to  the  view.  To  travelers,  sailors, 
and  meteorologists,  this  is  a  practice  that  has  become  familiar ;  but  very 
frequently  non-scientific  eyes  fail  to  distinguish  these  details. 

Yet,  in  some  cases,  and  especially  in  certain  regions  of  the  globe,  the 
mirage  is  so  plainly  evident  that  it  arrests  the  most  inattentive  gaze. 
Such  is  at  times  the  mirage  upon  the  coasts  of  the  Gulf  of  Messina;  and 
such,  it  appears,  is  that  seen  upon  the  sandy  plains  of  Arabia  and 
Egypt. 

The  mirage  is  sometimes  visible  upon  the  surface  of  the  sea,  and  of 
lakes  and  large  streams;  sometimes  upon  the  great  dry  and  sandy 
plains,  or  upon  high-roads  or  the  sea-shore. 

Very  frequently  these  misleading  appearances,  due  to  the  action  of 
solar  rays,  and  to  their  prismatic  reflection  across  the  strata  of  air  of 
unequal  density,  present  purely  imaginary  shapes  which  one  is  inclined 
to  consider  as  real,  although  their  origin  is  as  fortuitous  as  that  of  the 
appearances  occasionally  seen  in  the  clouds.  The  same  may  be  said  of 
those  unknown  islands  which  rise  up  in  mid-ocean  before  the  astonished 
navigator,  and  which  lead  him  astray  toward  imaginary  lands.  The 
Swedish  sailors  for  a  long  time  went  in  search  of  a  magic  island  that 
seemed  to  rise  between  those  of  Aland  and  of  Upland ;  it  turned  out  to 
be  only  a  mirage.  The  towns  which  seem  evolved  by  the  wand  of  a 
fairy  are  sometimes  but  the  reflection  of  distant  towns ;  but  more  fre- 
quently there  is  nothing  to  explain,  if  not  their  nature,  at  least  their 
origin.  M.  Grellois  says,  "  During  the  summer  of  1847, 1  was  proceed- 
ing one  very  hot  day  on  horseback,  at  a  walking  pace,  between  Ghelma 


15_j.  THE  ATMOSPHERE. 

and  Bone,  in  company  with  a  young  friend  who  has  since  died.  When 
we  had  arrived  within  about  two  leagues  of  Bone,  toward  one  in  the 
afternoon,  we  were  suddenly  brought  to  a  halt  at  a  turn  in  the  road  by 
the  appearance  of  a  marvelous  picture  unfolded  before  our  eyes.  To 
the  east  of  Bone,  upon  a  sandy  stretch  of  ground  which  a  few  days  be- 
fore we  had  seen  arid  and  bare,  there  rose  at  this  moment,  upon  a  gently 
sloping  hill  running  down  to  the  sea,  a  vast  and  beautiful  city,  adorned 
with  monuments,  domes,  and  steeples.  The  illusion  was  so  complete, 
that  reason  refused  to  admit  that  this  was  only  a  vision  which  held  us 
entranced  for  nearly  half  an  hour.  Whence  came  this  apparition? 
There  was  no  resemblance  to  Bone,  still  less  to  La  Calle  or  Ghelma, 
both  distant  twenty  leagues  at  least.  Are  we  to  suppose  it  was  the  re- 
flected image  of  some  large  city  on  the  Sicilian  coast?  That  seems  to 
me  very  improbable." 

The  inferior  mirage  is  sometimes  affected  by  simple  effects  of  refrac- 
tion, by  a  change  or  magnifying  of  the  objects  observed.  Thus,  for  in- 
stance, in  May  of  1837,'during  the  Algerian  expedition,  which  preceded 
the  treaty  made  with  Abd-el-Kader,  M.  Bonnefont  observed,  among  oth- 
ers, the  curious  mirage  described  below : 

"A  flock  of  flamingoes,  birds  of  prey  which  are  very  common  in  this 
province,  were  seen  upon  the  south-east  bank,  about  three  miles  and  a 
half  off.  These  birds,  as  they  left  the  ground  to  fly  to  the  surface  of 
the  lake,  assumed  such  enormous  dimensions  as  to  give  the  idea  of 
Arab  horsemen  defiling  one  after  the  other.  The  illusion  was  for  a 
moment  so  complete,  that  General  Bugeaud  sent  a  Spahi  forward  as  a 
scout.  The  latter  crossed  the  lake  in  a  straight  line;  but  when  he  had 
reached  a  point  where  the  undulations  commenced,  the  horse's  legs  be- 
came so  elongated,  that  both  steed  and  rider  seemed  to  be  borne  up  by 
a  fantastic  horse  several  yards  high,  and  disporting  itself  in  the  midst 
of  the  water  that  appeared  to  submerge  it.  All  eyes  were  fixed  on  this 
curious  phenomenon,  until  a  thick  cloud  intercepting  the  sun's  rays 
caused  these  optical  illusions  to  disappear,  and  re-established  objects  in 
their  natural  shape. 

"  Sometimes  another  effect,  which  became  a  source  of  amusement  to 
the  soldiers,  was  produced.  If,  while  the  sun  was  in  the  east  and  the 
wind  blowing  from  an'  opposite  direction,  a  small  and  buoyant  object, 
susceptible  of  being  floated  along  by  the  wind,  was  cast  into  the  lake, 
it  was  curious  to  observe  how  it  became  larger  as  it  got  farther  off,  and, 
as  soon  as  the  wind  had  made  it  undulate,  it  suddenly  took  the  shape  of 


THE  Milt  AGE.  155 

a  small  boat,  the  movement  of  which,  above  the  waves,  was  in  propor- 
tion to  the  shaking  it  experienced  from  the  wind.  The  objects  that  an- 
swered best  for  the  experiment  were  thistle-heads,  as  they  were  most 
easily  influenced,  even  by  the  lightest  breeze,  and  rendered  the  illusion 
complete.  At  about  half-past  eight  on  the  morning  of  June  18,  with  a 
temperature  of  26°  centigrade,  while  a  somewhat  strong  breeze  was 
blowing  from  the  east,  and  a  nebulous  stratum  was  beginning  to  dissi- 
pate the  heat,  a  certain  number  of  these  thistle-heads  were  launched 
upon  the  water,  and  no  sooner  had  the  wind  driven  them  to  the  point 
where  undulation  commenced,  than  they  presented  the  curious  spectacle 
of  a  fleet  in  disorder.  The  vessels  seemed  to  dash  one  against  the  other, 
and  then,  driven  by  the  wind  to  a  great  distance,  they  disappeared  as 
completely  as  if  they  had  gone  down." 

We  now  come  to  a  second  kind  of  mirage  which  is  often  seen,  but  the 
effects  of  which  are  less  striking,  and  which  has  consequently  been  less 
studied,  viz.,  the  approach  of  objects  situated  beyond  the  horizon,  and 
which  are  raised  above  it.  In  the  ordinary  mirage  which  we  have  just 
described,  the  density  of  the  air  increases  with  the  height,  the  trajecto- 
ries being  convex  toward  the  earth,  at  least  in  their  lower  parts.  In 
the  case  under  consideration,  the  density  decreases  and  the  trajectories 
become  very  concave  toward  the  ground.  A  luminous  ray,  at  first  hor- 
izontal, should,  as  it  moves  through  the  void,  remain  rectilinear;  but 
the  ordinary  atmospheric  refraction  inflects  this  trajectory,  imparting  to 
it  about  a  twelfth  part  of  the  terrestrial  curvature.  But  if  the  condition 
of  the  strata  is  modified,  and  if,  by  the  effect  of  an  abnormal  increase  in 
the  temperature,  the  density  decreases  with  the  height  much  more  than 
is  usual,  the  refracting  effect  of  these  strata  may  impart  to  these  traject- 
ories a  greater  curvature,  amounting  to  a  quarter,  a  half,  or  even  the 
whole  of  the  curvature  of  a  great  circle  of  the  earth.  Indeed,  some- 
times this  action  may  cause  it  to  exceed  this  latter  limit. 

In  these  fresh  conditions,  the  various  trajectories  passing  through  the 
eye  and  situated  in  the  same  vertical  plane,  instead  of  cutting  each  other 
two  and  two,  as  in  the  case  of  an  ordinary  mirage,  generally  diverge. 
Hence  it  results  that  we  can  not  obtain  two  reflections  of  one  object: 
If  the  depression  of  the  apparent  horizon  is  measured,  it  is  found  to  be 
very  much  raised,  sometimes  to  the  level  of  the  rational  horizon;  and 
objects,  usually  invisible  by  reason  of  their  great  distance  and  curva- 
ture of  the  earth,  may  become  visible.  The  accidental  position  of  these 
objects  beyond  the  apparent  contour  of  the  visible  horizon  makes  them 


156  THE  ATMOSPHERE. 

appear  to  be  much  nearer  than  usual,  while  another  circumstance  fa- 
vors the  illusion,  viz.,  the  transparency  of  the  air  during  the  occur- 
rence of  the  phenomenon.  It  is  clear  that,  as  no  reversal  of  the  ob- 
jects takes  place,  one  would  be  less  struck  with  this  particular  form  of 
mirage  than  with  that  which  corresponds  to  the  cases  previously  de- 
scribed. Woltmann  and  Biot  point  out  that  when  the  atmosphere  is 
in  this  particular  condition  the  sea  seems  to  be  concave,  at  the  same 
time  the  horizon  is  seen  above  the  hulls  of  ships,  distant  shores  take 
the  shape  of  high  cliffs,  and  very  distant  objects  seem  to  rise  in  the  air 
like  clouds. 

An  optical  circumstance  well  worthy  of  attention  is  the  following :  at 
the  same  time  that  some  objects  are  thus  raised  above  others  by  which 
they  are  ordinarily  hidden  from  view,  or  when  they  are  apparently 
removed  to  this  side  of  the  apparent  horizon,  they  seem  to  the  eye  to  be 
very  much  nearer.  Heim  has  described  a  case  of  this  kind  observed  in 
the  mountains  of  Thuringia,  where  he  suddenly  beheld  three  lofty  peaks 
appear  above  an  intermediate  chain  which  generally  concealed  them 
from  sight ;  and  these  peaks  appeared  to  be  so  clearly  defined  that  he 
was  able  to  distinguish,  with  an  ordinary  glass,  tufts  of  grass  that  were 
distant  four  German  miles.  M.  de  Tessan  saw  a  phenomenon  of  the 
same  kind  in  the  harbor  of  San  Bias  (California). 

A  letter  from  Teneriffe,  published  in  the  Courrier  des  /Sciences,  states 
that  from  the  summit  of  this  mountain,  whence  the  view  embraces  a 
horizon  of  fifty  leagues  radius,  a  mirage  rendered  visible  the  Alleghany 
Mountains  in  North  America,  a  thousand  leagues  distant.  I  scarcely 
can  venture  to  credit  this  story. 

Having  explained  the  two  great  classes  of  facts  relating  to  the  phe- 
nomena of  mirages,  one  of  which  is  due  to  the  depression  of  the  objects, 
and  the  other  to  their  elevation,  we  now  come  to  the  consideration  of 
another  effect  scarcely  less  curious,  viz.,  the  superior  mirage. 

This  presents  three  different  aspects.  Sometimes  the  reflection  is  seen 
inverted  above  the  object,  and,  above  the  former,  a  second  reflection, 
erect  as  the  object;  sometimes  the  first  reflection  alone  is  seen,  the  up- 
per one  having  disappeared ;  and,  thirdly,  the  upper  reflection  remains 
without  any  inverted  reflection  beneath  it. 

Woltmann  noticed  the  superior  mirage  on  three  different  occasions; 
objects  appeared  to  be  reflected  in  the  sky ;  in  the  air  was  seen  the  re- 
flection of  the  horizon  of  the  waters,  and  below  were  suspended,  upside 
down,  the  shores,  houses,  trees,  hills,  and' windmills.  Frequently  a  lay- 


THE  MIRAGE.  159 

er  of  air  separated  the  objects  turned  upside  down  from  those  beneath, 
but  usually  the  reflection  and  the  object  were  in  contact. 

Welterling  made  analogous  observations  upon  the  Svenska-Hogar, 
islands  situated  at  the  entrance  of  the  harbor  of  Stockholm.  He  says : 
"Above  each  of  the  sand-banks  a  black  spot  rises  and  appears  in  the 
air;  these  spots  then  become  elongated  downward,  and  finally  reach  the 
sand-bank,  which  assumes  the  appearance  of  a  column  nine  or  ten  times 
higher  than  it  really  is.  Hence  there  results  a  mock  horizon,  to  which 
all  the  objects  are  transported,  all  thus  appearing  in  a  straight  line  upon 
the  same  level,  though  their  absolute  height  differs  considerably." 

Crauz  saw  in  Greenland  the  shores  of  the  Kokernen  Islands,  raised  in 
the  shape  of  high  cliffs,  ancient  towers,  and  ruined  edifices.  Brandes 
several  times  witnessed  the  superior  mirage ;  as  a  rule  the  reflection  of 
objects  were  not  seen  very  distinctly  by  him,  for  he  adds  that  the  upper 
or  direct  reflection  was  generally  wanting,  and  he  attributes  this  fact  to 
the  want  of  spherical  shape  in  the  homogeneous  strata.  He  also  re- 
marks that  this  is  a  very  local  phenomenon,  being  seen  often  upon  the 
houses  in  the  eastern  part  of  Darngast,  and  at  the  same  time  being  invis- 
ible upon  those  in  the  west  part  of  the  town. 

In  December,  1869,  between  the  hours  of  three  and  four  in  the  morn- 
ing, a  mirage  was  seen  in  Paris,  as  represented  in  the  opposite  plate. 

These  objects  are  occasionally  delineated  in  the  sky  at  a  considerable 
height  above  the  horizon.  Some  move  very  rapidly,  and  others  are 
stationary,  while  they  are  sometimes  tinged  with  colors.  In  proportion 
as  the  light  augments,  the  shape  becomes  more  airy,  and  they  vanish 
entirely  when  the  sun  is  shining  with  full  brightness.  Mirage  may  also 
be  produced  by  two  strata  of  air  separated  by  a  vertical  plane.  This 
notably  occurs  in  the  case  of  large  walls  with  a  southern  aspect,  when 
they  are  heated  by  the  sun,  and  then  the  ordinary  mirage  is  formed. 
It  is  in  this  case  termed  the  lateral  mirage.  The  wall  in  this  instance 
acts  in  the  same  way  as  the  soil  when  exposed  to  the  solar  rays,  and  a 
line  perpendicular  to  the  wall  replaces  the  vertical  line  in  the  horizontal 
mirage.  But,  as  the  heated  strata  of  air  are  easily  renewed  as  they 
rise  along  the  wall,  the  disturbing  influence  of  the  densities  does  not  ex- 
tend very  far.  The  eye  must,  therefore,  be  placed  in  front  of  the  plane 
of  the  wall,  and  must  view  in  a  parallel  direction  any  objects  that  may 
approach  and  recede.  The  persons  who  approach  the  doors  in  the  wall, 
the  images  which  cross  in  the  sky  the  vertical  parallel  to  that  of  the 
wall,  are  always  seen  inverted,  as  indicated  in  the  theory  of  the  ordinary 


jgQ  THE  ATMOSPHERE. 

mirao-e.  Gruber  seems  to  have  been  one  of  the  earliest  spectators  of 
this  phenomenon.  Blackader  has  described  a  lateral  mirage  that  he 
saw  upon  a  wall  at  Leith.  It  was  also  observed  by  Gilbert. 

Let  us  add  to  the  above  the  multiplied  mirage  which  is  seen  when 
several  reflections,  all  inverted,  are  superposed  upon  the  object.  Biot 
and  Arago  saw  phenomena  of  this  kind  from  the  mountain  Desserto  de 
las  Palmas,  and  observed  at  night,  with  the  repeating  circle,  an  illumi- 
nated reflector  in  the  island  of  Ivyza.  Besides  the  ordinary  reflection, 
two,  three,  or  even  four  false  reflections,  superposed  in  the  same  vertical 
line,  have  been  seen.  Scoresby  observed,  on  July  18,  1822,  a  brig  with 
three  reflections  superposed,  all  inverted,  and  in  each  of  them  the  ves- 
sel was  in  contact  with  the  reflection,  also  inverted,  of  the  field  of  ice 
beyond  which  it  was  situated. 

The  mirage  does  not  always  present  such  regular  characteristics  as 

we  have  indicated;  sometimes 
the  second  reflection  is  seen 
above  the  original  one ;  some- 
times the  two  are  seen  beside 
each  other ;  and,  lastly,  the  re- 
flections sometimes  are  not  in- 
verted. 

Dr.  Vince  relates  several  re- 
markable observations.  From 
Ramsgate,  in  fine  weather,  may 
be  seen  the  tops  of  the  four 

Fig.  42.— Lateral  mirage  seen  on  Lake  Geneva.  i 

highest  towers  of  Dover  Castle. 

The  remainder  of  the  edifice  is  concealed  by  a  hill,  which  is  about 
twelve  miles  from  Eamsgate.  On  the  6th  of  August,  1866,  Dr.  Yince, 
looking  toward  Dover  at  seven  in  the  evening,  perceived,  not  only  the 
four  towers  as  usual,  but  the  entire  castle  from  roof  to  base,  as  dis- 
tinctly as  if  it  had  been  transported  to  the  hill  near  Ramsgate. 

In  the  polar  regions,  the  action  of  refraction  is  seen  under  the  most 
capricious  and  extraordinary  conditions.  Admiral  Wrangell  writes: 
"The  extreme  condensation  of  the  air  in  winter,  and  the  vapor  diffused 
in  the  atmosphere  in  summer,  give  great  power  to  refraction  in  the  fro- 
zen sea.  In  these  circumstances  the  mountains  of  ice  often  assume  the 
most  grotesque  shapes ;  sometimes,  indeed,  they  seem  to  be  detached 
from  the  icy  surface  which  serves  as  their  base,  so  as  to  appear  to  be 
suspended  in  the  air." 


THL  J 

Very  frequently  Admiral  Wrangell  and  his  companions  thought  they 
perceived  mountains  of  a  bluish  color,  whose  shapes  were  clearly  de- 
fined, and  between  which  they  thought  they  could  discern  valleys  and 
even  rocks.  But  just  as  they  were  congratulating  themselves  on  hav- 
ing discovered  the  long-sought  land,  the  bluish  mass,  carried  away  by 
the  wind,  extended  on  each  side,  and  finally  embraced  the  whole  hori- 
zon. Scoresby,  who  collected  so  much  interesting  information  in  these 
Greenland  regions,  has  also  pointed  out  that  ice  assumes  at  the  horizon 
the  most  regular  shapes,  and  even  appears,  at  many  points,  suspended 
in  the  air. 

The  most  curious  phenomenon  was  to  see  the  reflection,  inverted  and 
very  distinct,  of  a  vessel  below  the  horizon.  He  says:  "We  had  al- 
ready observed  similar  apparitions,  but  this  one  was  peculiar  for  the 
distinctness  of  the  reflection,  in  spite  of  the  great  distance  of  the  vessel. 
Its  contour  was  so  well  defined  that,  in  looking  at  it  with  a  Dolland's 
glass,  I  could  distinguish  the  details  of  the  masts  and  the  hull  of  the 
ship,  which  I  recognized  as  that  of  my  father.  On  comparing  our 
books,  we  saw  that  we  were  34  miles  from  each  other,  that  is,  19^  miles 
from  the  horizon,  and  far  beyond  the  limits  of  vision." 

Upon  the  shores  of  the  Orinoco,  Hurnboldt  and  Bonpland  discovered 
that  at  noon  the  temperature  of  the  sand  was  127°,  while  at  six  yards 
above  the  ground  the  temperature  of  the  air  was  only  104°.  The  hil- 
locks of  San  Juan  and  Ortez,  the  chain  called  the  Oalera,  situated  three 
or  four  leagues  off,  seemed  suspended  in  the  air;  the  palm-trees  ap- 
peared to  have  no  hold  on  the  ground,  and,  in,  the  midst  of  the  savanna 
of  Caraccas,  these  savans  saw,  at  a  distance  of  a  mile  and  a  half,  a  herd 
of  oxen  apparently  in  the  air.  They  noticed  no  double  reflection.  Hum- 
boldt  also  remarked  a  herd  of  wild  cattle,  part  of  which  seemed  to  be 
above  the  surface  of  the  ground,  while  the  remainder  were  standing 
upon  the  soil. 

Mirages  are  not  exclusively  phenomena  of  warm  climates;  as  we 
have  seen,  they  have  been  observed  in  the  very  heart  of  the  polar  seas. 

When,  instead  of  occurring  in  plane  and  regular  strata,  refractions 
and  reflections  take  place  in  the  curved  and  irregular  strata,  a  mirage  is 
produced,  the  reflections  of  which  are  deformed  in  all  directions,  broken 
or  repeated  several  times,  and  very  far  distant  from  one  another. 

This  is  the  case  with  the  fantastic  aerial  vision,  formerly  attributed 
to  a  fairy — the  Fata  Morgana — which  sometimes  attracts  crowds  of  peo- 
ple to  the  sea-shore  at  Naples  and  at  Reggio,  upon  the  Sicilian  coast. 

11 


162 


THE  ATMOSPHERE. 


The  phenomenon  generally  occurs  of  a  morning  in  very  calm  weather. 
For  an  extent  of  several  leagues  the  sea  upon  the  Sicilian  coast  assumes 
the  appearance  of  a  chain  of  sombre  mountains,  while  the  waters  upon 
the  Calabrian  side  remain  quite  unaffected.  Above  the  latter  is  seen 
depicted  a  row  of  several  thousands  of  pilasters,  all  of  equal  elevation, 
of  equal  distance  apart,  and  of  equal  degrees  of  light  and  shade.  In 
the  twinkling  of  an  eye  these  pilasters  sometimes  lose  half  their  height, 
and  appear  to  take  the  shape  of  arcades  and  vaults,  like  the  Koman 


Fig.  43.— La  Fata  Morgana. 

aqueducts.  There  is  often,  also,  noticeable  a  long  cornice  upon  their 
summits,  and  there  are  also  seen  countless  castles,  all  exactly  alike. 
These  soon  fade  away,  and  give  place  to  towers  which  in  turn  disap- 
pear, leaving  nothing  but  a  colonnade,  then  windows,  and  lastly  pine- 
trees  and  cypresses,  several  times  repeated. 

Similar  fantastic  apparitions  were  noticed  with  great  surprise  in  the 
neighborhood*,©/  Edinburgh  on  the  16th  and  17th  of  June,  1870,  pre- 
vious to  a  severe  thunder-storm.  These  are  unquestionably  among  the 
most  curious  kinds  of  mirage  that  exist. 


SHOOTING- STARS.  163 


CHAPTER  VII 

SHOOTING-STARS — BOLIDES  —  AEROLITES  —  STONES  FALLING  FROM 
THE   SKY. 

NONE  of  my  readers  will  have  failed  to  have  been  struck  with  sur- 
prise, during  the  calm  of  a  fine  starry  night,  by  the  spectacle  of  a  star 
gliding  noiselessly  through  the  celestial  vault  to  extinction.  Some, 
perhaps,  of  those  who  peruse  these  pages,  may  have  enjoyed  the  rare 
privilege  of  beholding,  not  only  a  shooting -star,  but  a  more  brilliant  and 
sometimes  very  exciting  phenomenon,  viz.,  the  rapid  passage  through 
space  of  a  flaming  bolide,  scattering  a  gleaming  light  in  all  directions — 
a  globe  of  fire,  leaving  a  luminous  track  behind  it,  and  sometimes  burst- 
ing with  an  explosion  like  that  of  an  enormous  shell,  and  a  report  like 
that  of  a  cannon.  Some,  perhaps,  also,  by  a  still  more  fortunate  chance, 
have  had  an  opportunity  of  picking  up  a  fragment  of  an  exploded  bo- 
lide— a  piece  that  has  fallen  from  the  sky — an  aerolite  or  stone  that  has 
come  down  from  the  heights  of  the  atmosphere. 

We  here  have  three  distinct  facts,  which  nevertheless  seem  to  be  re- 
lated to  each  other  in  their  origin.  The  progress  made  during  the  last 
few  years  in  the  special  study  of  these  meteors  is  a  reason  for  consider- 
ing them  separately,  taking  first  the  shooting-stars,  then  the  bolides,  and 
lastly  the  aerolites. 

The  first  point  to  consider  in  the  study  of  shooting-stars  is  the  meas- 
urement of  the  height  at  which  they  are  seen.  Two  spectators,  placed 
at  a  distance  of  some  miles  from  each  other,  notice  the  passage  of  a 
shooting  -  star  among  the  constellations;  its  path  is  not  exactly  the 
same  to  both  observers,  owing  to  perspective.  From  the  observation 
of  these  two  paths  the  distance  can  be  obtained.  This  method,  as  early 
as  1798,  two  German  savans,  Brandes  and  Benzemberg,  had  already 
made  use  of.  From  the  latest  researches  upon  this  head  made  by  Al- 
exander Herschel  (grandson  of  the  famous  Sir  William  Herscnel),  by 
Professor  Newton,  of  New  Haven,  Conn.,  and  by  Father  Secchi,  Director 
of  the  Observatory  at  Rome,  it  has  been  concluded  that  the  average 
height  of  a  shooting-star  is  seventy-five  miles  when  first  seen,  and  fifty 
miles  at  the  end  of  its  visible  journey. 


164:  THE  ATMOSPHERE. 

The  velocity  varies  from  seven  to  forty  miles  a  second. 

Shooting-stars  are  not  common  to  all  nights  of  the  year  alike,  for  the 
result  of  observations  shows  that  there  are  yearly,  monthly,  and  daily 
periods  of  recurrence  of  certain  sets  of  shooting-stars.  Great  showers 
of  shooting-slprs  on  particular  nights  have  been  remarked  since  the  last 
century ;  Brandes  relates  that,  on  December  6, 1798,  during  a  carriage- 
drive  to  Bremen,  he  counted  four  hundred  and  eighty  from  the  coach 
window ;  and  he  estimates  that,  at  that  rate,  there  must  have  been  at 
least  two  thousand  in  the  course  of  the  night. 

During  the  night  of  the  llth  to  the  12th  of  November,  1799,  Hum- 
boldt  and  Bonpland  witnessed  a  perfect  shower  of  shooting-stars  at  Cu- 
mana  (South  America).  Bonpland  states  that  there  was  no  part  of  the 
sky  equal  in  extent  to  three  diameters  of  the  moon  that  was  not  con- 
tinuously being  filled  with  shooting-stars.  The  inhabitants  of  Cumana 
were  terrified  by  this  phenomenon,  and  the  oldest  of  them  remembered 
an  analogous  occurrence  in  1766,  accompanied  by  an  earthquake. 

This  shower  of  stars  at  the  close  of  the  last  century  had  been  nearly 
forgotten,  when  a  fresh  shower  was  seen  in  America  on  November  13, 
1833.  Professor  Olmsted,  of  New  Haven,  Conn.,  basing  his  calculations 
upon  data  which  had  been  transmitted  to  him,  regards  the  number  of 
shooting-stars  that  appeared  in  certain  districts  on  that  occasion  as  over 
two  hundred  thousand.  Olmsted  was  the  first  to  point  out  that  the 
great  display  of  November  must  be  periodical,  and  would  be  reproduced 
every  year  at  the  same  epoch.  A  very  considerable  increase  in  the 
number  of  shooting-stars  at  that  date  has,  in  fact,  been  noticed,  but  not 
to  the  extent  of  the  extraordinary  phenomenon  in  America  in  1833. 
The  astronomer  Olbers,  writing  on  the  same  subject  in  1837,  says :  "We 
shall,  perhaps,  have  to  wait  until  1867  for  the  recurrence  of  the  splendid 
phenomenon  witnessed  in  1799  and  1833."  This  bold  prediction  was 
completely  realized  just  a  twelvemonth  earlier,  in  1866. 

From  a  general  discussion  of  the  observations,  it  results  that  the  num- 
ber of  shooting-stars  which  ordinarily  appear  over  the  whole  extent  of 
the  visible  sky  in  the  space  of  an  hour  is,  on  an  average,  from  ten  to 
eleven. 

Now,  at  the  time  of  the  maximum  on  November  12  and  13,  this 
hourly  number,  which  was  equal  to  fifty  in  1834,  gradually  fell  annual- 
ly, until  it  was  reduced  to  thirty  in  1839,  to  twenty  in  1844,  to  seven- 
teen in  1849 ;  three  or  four  years  later  the  maximum  had  disappeared, 
and  was  replaced  by  a  normal  appearance  of  from  ten  to  eleven  an  hour 


SHOOTING-STARS.  165 

Matters  remained  in  this  condition  until  1863,  when  a  maximum  of 
thirty-seven  an  hour  again  occurred  at  the  same  epoch,  rising  to  seventy- 
four  an  hour  the  next  year,  and  thus  acting  as  a  precursor  of  the  great 
phenomenon  of  1866,  when  Olbers's  prediction  was  fulfilled.  Another 


Fig.  44— Shooting-stars. 

maximum  occurred  on  August  10,  and  was  noticed  by  M.  Quetelet  so 
long  ago  as  1837.  The  maximum  hourly  number  of  shooting-stars  was, 
on  that  night,  fifty-nine.  There  was  a  progressive  rise  in  the  number 
to  seventy-nine  in  1841,  to  eighty-five  in  1845,  and  to  one  hundred  and 
ten  in  1848,  from  which  date  it  gradually  decreased  each  year,  standing 
at  thirty-eight  in  1859,  since  which  time  it  has  alternately  risen  and 
fallen,  varying  between  the  numbers  thirty-seven  and  sixty-seven. 

Here  we  have  a  well-ascertained  annual  variation  in  these  periodical 
showers.  The  researches  of  Coulvier-Gravier  clearly  establish  the  ex- 
istence of  a  monthly  variation,  the  number  of  shooting-stars  being  great- 
er in  autumn  than  in  spring.  There  is,  also,  a  daily  variation.  The 
hourly  numbers,  from  six  in  the  evening  to  six  in  the  morning,  are 
twice  as  great  as  for  the  corresponding  hours  in  the  day-time. 

Shooting-stars  are  seen  in  all  parts  of  the  sky ;  but  if  the  directions 
whence  they  seem  to  come  are  examined,  it  is  found  that  the  different 


Igg  THE  ATMOSPHERE. 

parts  of  the  horizon  furnish  different  numbers.  There  is  thus  a  varia- 
tion in  this  respect  which  is  termed  the  azimuihal  variation,  and  which 
has  been  thoroughly  studied  by  means  of  carefully  registered  observa- 
tions. Many  more  shooting-stars  come  from  the  east  than  from  the 
west,  but  nearly  equal  numbers  from  the  north  and  the  south. 

At  the  periods  of  the  maxima,  toward  the  12th  and  13th  of  Novem- 
ber, and  toward  the  9th  and  10th  of  August,  the  shooting-stars,  instead 
of  appearing  in  all  the  regions  of  space  indifferently,  nearly  all  come 
from  given  directions.  Some  (those  of  November)  start  from  the  con- 
stellation Leo;  the  others  (August)  emanate  from  the  constellation  Per- 
seus. What  path  in  space  is  then  taken  by  these  periodical  showers, 
the  existence  of  which  is  ascertained  ? 

It  has  been  observed  that  the  speed  of  the  meteors  is  equal  to  that  of 
comets  descending  toward  the  earth  from  the  depths  of  space,  and  their 
orbit  has  been  also  assimilated  to  the  orbits  of  the  comets.  Signer  Schi- 
aparelli,  Director  of  the  Milan  Observatory,  sought  to  determine  the  ele- 
ments which  characterize  the  shape  and  the  position  of  the  apparent 
parabola  followed  by  the  meteoric  current  of  the  10th  of  August.  He 
then  compared  these  astronomical  elements  with  those  obtained  by  cal- 
culating the  orbits  of  the  different  comets.  He  was  thus  able  to  estab- 
lish a  very  unexpected  similarity  between  the  orbit  that  he  had  just  dis- 
covered for  the  swarm  of  shooting-stars  of  the  10th  of  August  and  that 
of  the  great  comet  seen  in  1862. 

Supposing  that  every  one  hundred  and  eight  years  these  meteors 
have  a  frequency  neither  so  sudden  nor  so  short  in  duration  as  that  of 
November  meteors,  but  lasting  twenty  or  thirty  years,  this  period  agrees 
with  the  duration  of  the  revolution  of  the  great  comet  of  1862,  and  may 
be,  therefore,  taken  to  represent  that  of  the  successive  returns  of  the 
comet  to  its  perihelion. 

M.  Schiaparelli  then  set  to  work  to  discover  the  elements  of  the  orbit 
of  the  November  swarm  of  shooting-stars.  Observation  in  this  instance 
supplied  him  with  further  data;  the  period  of  return  for  the  great  dis- 
plays of  November,  indicated  by  Olbers  in  1837,  had  just  been  confirmed 
in  1866,  and  might  be  fixed  at  thirty-three  years  and  a  fraction.* 

*  [Taking  as  data  the  observed  directions,  etc.,  of  the  November  meteors,  the  researches  of 
Professors  Newton  (New  Haven,  Conn.)  and  Adams  have  shown  that  their  orbit  must  be  an 
ellipse,  the  periodic  time  of  which  is  about  33^  years,  agreeing  exactly  with  observation.  A 
small  discrepancy  has  also  been  satisfactorily  explained  as  the  result  of  the  attraction  of  the 
larger  planets,  especially  Jupiter.— ED.] 


BOLIDES.  167 

A  swarm  of  shooting-stars,  seen  on  the  10th  of  December,  describes 
in  space  the  same  ellipse  as  the  well-known  Biela's  comet,  and  the  shoot- 
ing-stars seen  on  the  20th  of  April  move  along  the  orbit  of  the  first 
comet  of  1861.  Such  researches  have  thrown  a  great  light  upon  the 
question  of  shooting-stars.  The  comet  which  traces  in  space  the  same 
path  as  the  swarm  of  meteors  must  be  considered  as  an  integral  part  of 
it.  It  is,  in  fact,  merely  a  local  concentration  of  the  matter  of  the  swarm 
— a  concentration  so  intense  that  the  mass  of  matter  it  forms  is  visi- 
ble even  at  a  great  distance  from  the  earth.  According  to  this  theory, 
shooting-stars  are  of  the  same  nature  as  comets,  consisting  of  small  neb- 
ulous objects  which  move  in  space  without  being  visible  to  us  because 
of  their  smallness,  and  only  becoming  so  when  they  penetrate  into  the 
atmosphere  of  the  earth.  Like  comets,  they  seem  to  be  gaseous. 

A  current  of  these  meteors  which  encounters  the  orbit  of  the  earth 
at  a  certain  point,  and  the  different  parts  of  which  take  several  years  to 
pass  this  point  of  meeting,  must  be  crossed  by  the  earth  each  year  at 
the  same  epoch.  Hence  the  periodical  showers  of  shooting-stars  which 
are  reproduced  from  year  to  year,  with  varying  intensity,  according  to 
the  greater  or  less  concentration  of  the  nebulous  matter  in  the  various 
parts  of  the  current  which  the  earth  successively  reaches. 

Such  are  shooting-stars.  Now  we  come  to  the  Bolides.  If  shooting- 
.stars  are  gaseous,  there  is  an  essential  distinction  between  them  and 
bolides,  for  the  great  majority  of  the  latter  are  unquestionably  solid. 
To  give  an  idea  of  the  meteoric  phenomenon  of  the  explosion  of  a  bo- 
lide, I  will  cite,  among  the  most  recent  falls,  one  that  occurred  by  day 
and  another  that  occurred  at  night,  both  in  1868. 

This  is  the  account  of  the  fall  of  a  bolide  by  day,  which  took  place  in 
the  arrondissement  of  Casale,  in  Piedmont,  on  the  29th  of  February. 
It  was  half-past  ten  in  the  morning,  but  the  sky  was  rather  dark.  Sud- 
denly a  loud  detonation  was  heard,  similar  to  the  discharge  of  a  heavy 
piece  of  artillery,  or,  perhaps,  rather  to  the  explosion  of  a  mine.  This 
was  followed,  at  an  interval  of  two  seconds,  by  another  report  resulting 
from  two  distinct  detonations,  which  succeeded  each  other  so  closely 
that  the  second  seemed  to  be  the  continuation  or  the  prolongation  of  the 
first.  These  detonations  were  heard  as  far  off  as  Alexandrie,  a  distance 
of  twenty  miles.  The  sound  had  not  yet  died  away  when  there  became 
visible,  at  a  considerable  height  above  the  ground,  a  mass  irregular  in 
shape  and  enveloped  in  smoke,  thus  resembling  a  small  cloud.  It  left 
behind  a  long  train  of  smoke ;  other  spectators  saw  distinctly,  and  at  a 


168  THE  ATMOSPHERE. 

great  height,  not  one  but  several  spots  like  small  clouds  which  disap- 
peared nearly  instantaneously.  Some  men  at  work  in  the  fields  saw 
several  blocks  fall  through  the  air,  and  heard  the  noise  which  they  made 
as  they  struck  the  ground.  Every  one  whom  it  was  possible  to  ques- 
tion on  the  subject  was  unanimous  in  affirming  that  there  were  a  large 
number  of  these  blocks,  and  that  they  must  have  occasioned  a  regular 
shower  of  aerolites  of  all  sizes.  Laborers  at  work  felling  trees  in  a 
wood  three-quarters  of  a  mile  from  Villeneuve,  on  the  high-road  from 
Casale  to  Yercelli,  saw  something  like  a  hailstorm  of  grains  of  sand  after 
these  detonations,  and  a  somewhat  large  fragment  struck  the  hat  that 
one  of  them  was  wearing.  The  aerolites  found  upon  the  ground  con- 
sisted of:  1st,  a  piece  weighing  4^  Ibs.,  which  fell  in  a  wheat-field  650 
yards  to  the  south-east  of  Villeneuve,  and  penetrated  sixteen  inches 
into  the  ground ;  2d,  a  piece  weighing  14|-  Ibs.,  which  fell  in  a  newly- 
sown  field  to  the  north  of  Villeneuve,  7700  feet  from  the  first,  and  en- 
tered the  ground  to  a  depth  of  144  inches ;  3d,  the  numerous  frag- 
ments into  which  a  third  piece  broke  by  falling  upon  the  pavement  in 
front  of  the  inn  of  Molta  dei  Conti,  at  a  distance  of  10,335  feet  from 
the  first  piece,  and  of  10,630  feet  from  the  second. 

The  recital  of  the  nocturnal  fall  will  help  to  complete  the  comprehen- 
sion of  these  singular  occurrences.  It  took  place  in  the  arrondissement 
of  Mauleon,  in  the  Lower  Pyrenees,  on  September  7, 1868,  at  half-past 
ten  in  the  morning. 

The  sky  was  suddenly  illuminated  by  a  meteor,  which  looked  like  a 
burning  ball  with  a  long  train  of  fire  in  its  track.  It  emitted  a  bright 
light  of  a  pale  greenish  hue,  and  lasted  for  six  or  ten  seconds.  Its  dis- 
appearance was  preceded  by  an  explosion,  and  by  the  simultaneous  pro- 
jection of  flaming  fragments,  while  there  remained  for  some  time  after 
a  light  and  whitish  cloud.  This  was  followed  by  a  continuous  noise, 
like  the  distant  rolling  of  thunder,  then  by  three  or  four  detonations  of 
extreme  violence,  which  were  heard  at  points  distant  fifty  miles  from 
each  other.  Immediately  after  these  detonations  the  inhabitants  of 
Sanguis-Saint-fitienne  heard  a  hissing  noise  like  that  made  by  red-hot 
iron  when  it  is  plunged  into  water,  then  a  dull  sound  indicating  the  fall 
of  a  solid  body  to  the  ground.  The  mass  had  fallen  at  about  thirty 
yards  from  the  church  of  Sanguis,  in  the  bed  of  a  small  stream,  and  was 
shattered  into  fragments,  the  largest  of  which  was  scarcely  two  inches 
long.  The  fall  was  witnessed  by  two  men  who  were  talking  together, 
and  who,  terrified  at  the  detonations  and  the  hissing  noise,  had  thrown 


BOLIDES.  169 

themselves  upon  the  ground  just  as  the  stone  fell  about  twenty  paces 
before  them.  The  weight  of  the  stone  was  estimated  at  from  six  to 
eight  pounds. 

These  two  instances,  which  I  select  from  an  immense  number,  give 
a  sufficient  idea  of  these  downfalls  from  the  sky,  which  were  formerly 
looked  upon  as  fabulous.  It  is  only  in  the  last  half  century  that  the 
facts  have  been  credited  and  scientifically  confirmed. 

In  contradistinction  to  the  shooting-stars  which  become  extinguished 
and  lost  in  the  upper  regions,  the  bolides  traverse  all  the  atmospheric 
strata,  and  often  reach  the  surface  of  the  earth.  This  is  the  reason  why 
the  luminous  phenomena  that  accompany  them  usually  appear  to  us 
much  more  intense ;  because,  in  fact,  the  regions  in  which  they  occur 
are  much  nearer  to  us.  But  when  seen  from  afar,  as  is  the  case  with 
those  whose  directions  prevent  them  from  reaching  the  lower  strata  of 
the  atmosphere,  bolides  present  the  same  appearance  to  our  eyes  as 
shooting-stars.  When  they  do  reach  the  lower  air,  an  explosion,  sim- 
ple or  repeated,  often  takes  place,  followed  in  the  majority  of  cases  by 
a  fall  of  fragments  from  the  bolide  that  have  become  detached  from 
the  main  mass  by  the  effect  of  the  explosion.  Bolides,  then,  are  solid 
bodies,  like  the  fragments  detached  from  them.  The  orbits  described 
by  these  bolides,  in  their  movement  relative  to  the  earth,  have  some- 
times been  found  to  be  ellipses  of  such  limited  dimensions,  that  one 
would  be  led  to  suppose  that  the  former  were  nothing  but  satellites 
of  the  earth,  only  visible  during  their  passage  through  the  atmos- 
phere— a  view  adopted  by  Petit,  of  Toulouse.  On  the  other  hand, 
their  orbits  have  sometimes  been  found  to  be  hyperbolic  arcs,  nearly 
rectilinear,  and  traversed  with  great  speed  —  a  fact  tending  to  show 
that  bolides  possessing  such  rapid  movement  must  come  from  the 
stellar  regions. 

The  aerolites  are  minerals  that  fall  from  the  sky  to  the  earth.  They 
proceed  from  the  explosion  of  a  bolide. 

Sometimes  they  plunge  deeply  into  the  soil  upon  which  they  fall. 
Thus  the  island  of  Lanaia-Uawai  possesses  an  aerolite  six  or  seven 
yards  in  diameter,  which  has  remained  imbedded  in  the  ground  in  de- 
spite of  all  the  efforts  made  to  raise  it  to  the  surface.  This  aerolite  fell 
at  the  beginning  of  the  century.  (Very  recently,  on  the  9th  of  March, 
1868,  at  9-30  P.M.,  another  bolide  fell  upon  the  same  island.) 

These  stones,  if  touched  immediately  after  their  fall,  seem  to  be  burn- 
ing hot ;  but  they  cool  very  rapidly — a  fact  indicating  that  their  higher 


170  THE  ATMOSPHERE. 

temperature  was  altogether  superficial,  and  did  not  extend  to  the  inte- 
rior of  their  mass. 

As  to  the  shape  of  these  aerolites,  it  is  neither  that  of  a  ball,  more  or 
less  round,  nor  that  of  an  object  with  a  rounded  surface;  they  rather 
resemble  polyhedra,  with  rough,  irregular  sides  and  ridges.  The  plane 
parts  of  their  surface  have  often  hollows  analogous  to  those  produced 
by  the  pressure  of  a  round  body  upon  a  pasty  substance.  They  are, 
moreover,  enveloped  in  a  black  crust,  generally  of  a  dark  hue,  but  some- 
times lustrous,  as  if  covered  with  very  thin  varnish. 


Pig.  45.— Fall  of  a  bolide  in  the  day-time. 

The  light  displayed  in  the  movements  of  the  bolides  is  due  entirely 
to  the  heat  produced  by  the  compression  of  the  air.  Let  us  examine 
in  what  way  the  phenomena  of  explosion,  and  the  falls  of  the  aerolites 
which  often  succeed  it,  are  produced. 

The  enormous  compression  of  the  air  forced  back  by  the  bolide  can 
not  occur  without  this  air  reacting  upon  the  anterior  part  of  the  surface 
of  this  body,  and  exercising  a  considerable  pressure  upon  it.  Attrib- 
uting to  the  bolide  a  speed  of  four  and  a  half  miles  per  second— by  no 
means  an  exaggerated  estimate— M.  Haidinger  calculates  the  resisting 


BOLIDES.  171 

pressure  which  the  bolide  meets  with  from  the  air  at  more  than  twenty- 
two  atmospheres.  Such  a  pressure  evidently  tends  to  crush  the  body 
which  is  exposed  to  it ;  and  if  this  body,  in  its  more  or  less  irregular 
shape  and  constitution,  offers  portions  of  itself  which  are  more  opposed 
than  the  others  to  the  action  of  this  pressure,  these  portions  may  give 
way  and  become  suddenly  detached  from  the  mass  of  the  bolide. 

Broken  off  and  started  in  a  direction  contrary  to  that  in  which  they 
were  traveling  a  few  moments  before  with  the  main  mass  of  bolide, 
these  fragments  soon  lose  the  speed  with  which  they  were  endowed,  and 
reach  the  terrestrial  surface,  still  moving  with  very  great  velocity,  but 
not  with  the  rapidity  of  bodies  falling  to  the  earth  from  space. 

We  are  inclined  to  look  upon  the  bolides  as  being  somewhat  similar 
in  origin  and  being  to  the  planets  which  circulate  in  such  great  num- 
bers around  the  sun,  and  as  probably  themselves  forming  part  of  our 
planetary  system.  Besides,  the  discovery  recently  made  of  a  large 
number  of  planets  of  very  small  dimensions,  induces  us  to  believe  that 
there  exists  a  multitude  of  others  still  smaller  which  have  escaped  ob- 
servation. 

In  consequence  of  the  great  difficulties  that  were  encountered  in  at- 
tributing to  the  bolides  a  purely  terrestrial  origin,  it  was  long  ago  sug- 
gested that  they  might  be  stones  hurled  to  the  earth  from  the  volcanoes 
of  the  moon.  This  idea  was  taken  up  and  developed,  in  1795,  by  Olbers, 
and  in  the  early  part  of  the  present  century  by  Laplace,  Lagrange,  Pois- 
son,  and  Biot;  but  serious  objections  of  more  than  one  kind  soon  ap- 
peared to  render  this  theory  untenable,  and  it  was  finally  abandoned  for 
that  of  Chladni,  whose  system  consisted  in  regarding  the  bolides  as 
bodies  wandering  freely  in  space,  and  penetrating  every  now  and  then 
the  atmosphere  of  the  earth. 

Whatever  may  be  the  part  played  by  the  bolides  in  the  universe,  the 
possibility  afforded  us  of  examining  the  fragments  which  they  leave  in 
their  passage  is  very  useful  in  regard  to  the  information  which  we  are 
enabled  to  extract  from  them  as  to  the  constitution  and  nature  of  bodies 
foreign  to  the  globe  which  we  inhabit.  Thus  great  pains  have  been 
taken  of  late  years  to  collect  from  all  quarters  stones  that  have  fallen 
from  the  sky  after  the  explosion  of  bolides ;  and  collections  of  this  spe- 
cial kind  of  rock  have  been  made,  to  which,  in  order  to  distinguish 
them  from  the  terrestrial  rocks,  the  special  denomination  of  meteorites 
has  been  given.  There  are  at  various  places  beautiful  and  valuable 
collections  of  this  kind ;  among  others,  that  in  the  Museum  of  Natural 


172 


THE  ATMOSPHERE. 


History  in  Paris,  that  in  the  British  Museum,  and  that  in  the  Mineral- 
ogical  Museum  at  Vienna.  The  Paris  collection,  under  the  superintend- 
ence of  M.  Daubree,  contains  at  present  specimens  of  240  meteorites, 
while  all  the  known  falls  do  not  exceed  255. 

It  is  easy  to  understand  that  conflagrations  may  have  been  caused  by 
the  fall  of  aerolites,  and  that  people  may  have  been  killed  by  them. 
Fourteen  deaths  have  been  ascertained  to  have  taken  place  from  this 
cause  at  various  times. 

The  largest  stones  known  to  have  fallen  are  as  follows : 
The  aerolite  that  fell  at  Juvenas  in  the  Ardeche,  on  June  15,  1821, 
weighed  212  Ibs.,  exclusive  of  the  fragments  detached  from  it. 


Fig.  46.— The  Caille  aerolite,  weighing  12J  cwt 

The  aerolite  found  in  Chili,  between  Rio-Juncal  and  Padernal,  in  the 
Upper  Cordilleras  of  Atacama,  weighed  240  Ibs.,  and  was  in  the  shape 
of  a  cone,  measuring  nineteen  inches  in  length  and  eight  inches  in  di- 
ameter. The  miners  who  brought  it  home  upon  their  mules  had  taken 
it  for  a  block  of  silver.  It  was  in  the  Paris  Exhibition  of  1867. 

The  meteoric  stone  of  Murcia,  which  is  in  the  Museum  of  Natural 
Sciences  at  Madrid,  weighs  2£  cwt. 

The  aerolite  which  fell  in  1492,  at  Bnsisheim,  in  the  Upper  Ehine,  in 
the  presence  of  Maximilian  I,  king  of  the  Bomans,  weighs  2f  cwt. ;  it 


AEROLITES.  173 

is  imbedded  five  feet  in  the  ground,  and  was  long  venerated  by  the 
Church  as  a  miraculous  object. 

The  aerolite  that  fell  on  Christmas-day,  1869,  at  Mourzouk  (latitude 
26°  N.,  longitude  12°  E.  of  Paris),  in  the  midst  of  a  group  of  terrified 
Arabs,  must  weigh  much  more,  for  it  is  nearly  a  yard  in  diameter,  li 
is  to  be  taken  to  Constantinople,  but  will,  unfortunately,  have  to  be  pre- 
viously divided. 

None  of  these,  however,  approach  the  Caille  aerolite,  in  the  Maritime 
Alps,  which  was  used  as  a  seat  at  a  church  porch,  and  which  is  now  in 
the  Paris  Museum.  It  weighs  12£  cwt.  (see  Fig.  46). 

The  aerolite  that  fell  in  1810  at  Santa-Rosa  (New  Granada)  in  the 
night  of  April  20,  21,  weighs  14f  cwt.  When  found,  it  was  almost  im- 
bedded in  the  ground  by  the  force  of  the  fall. 

Lastly,  the  most  colossal  of  the  known  stones  that  have  fallen  from 
the  sky  is  the  aerolite  brought  back  from  the  Mexico  campaign,  weigh- 
ing more  than  15^  cwt.  It  had  from  time  immemorial  been  lying  at 
Charcas.  Its  shape  is  that  of  a  truncated  triangular  pyramid,  measuring 
a  yard  in  height,  and  it  is  a  fair  specimen  of  the  world  that  sent  it  to  us. 
From  several  hundred  analyses  made  by  the  most  eminent  chemists,  it 
appears  that  the  meteorites  have  added  no  single  substance  to  the  globe 
which  it  did  not  possess  before.  The  elements  up  to  this  time  discover- 
ed to  be  existent  in  them  are  twenty-two  in  number. 


174  THE  ATMOSPHEME. 


CHAPTER  VIII. 

THE   ZODIACAL  LIGHT. 

To  complete  the  panorama  of  the  optical  phenomena  of  the  sky,  we 
will  now  consider  the  nature  of  a  nocturnal  brightness  which  is  seen  in 
the  heights  of  the  atmosphere  on  certain  clear  nights.  As  in  the  case 
of  shooting-stars  and  bolides,  its  origin  is  in  the  depth  of  space,  and  the 
explanation  of  it  belongs  to  astronomy ;  but,  as  it  reveals  itself  in  our 
sky,  it  deserves  notice  in  these  pages. 

After  sunset  in  January,  February,  March,  and  April,  and  after  sun- 
rise in  November,  the  celestial  vault  sometimes  displays  a  band  of  light 
inclined  toward  the  horizon  and  in  the  plane  of  the  zodiac ;  that  is,  in 
the  apparent  path  that,  by  its  annual  change  of  position,  the  sun  seems 
to  trace  out  in  the  sky.  This  light  was  not  remarked  till  comparatively 
recently,  and  the  discovery  of  it  is  due  to  Childrey,  who  speaks  of  it  in 
his  "  Natural  History  of  England,"  published  about  1659.  The  earliest 
scientific  researches  with  regard  to  this  phenomenon  were  not,  howev- 
er, made  until  1683 ;  they  are  due  to  J.  D.  Cassini.  When  the  zodiacal 
light  first  appears  in  the  evening  after  sunset,  it  is  interfered  with 
near  the  horizon  by  the  last  traces  of  the  twilight  glimmer,  and  the 
union  of  these  two  lights  presents  the  appearance  of  a  cone.  This 
oblique  cone,  at  least  in  our  climates,  has  its  base  upon  the  horizon  and 
its  summit  at  a  certain  height  above. 

Toward  the  equator  this  brightness  rapidly  loses  its  conical  aspect  as 
the  last  traces  of  twilight  disappear,  and  when  night  has  fully  set  in  a 
band  of  light  may  be  distinguished  right  round  the  sky,  and  making 
the  zodiac  luminous,  so  to  speak ;  sometimes  this  band  is  visible  unin- 
terruptedly from  sunset  to  sunrise.  The  parts  nearest  to  the  sun  exceed 
in  brilliancy  the  intensity  of  the  Milky  Way ;  the  other  parts  are  dim, 
and  if  they  are  visible  at  all  in  the  intertropical  zone,  it  is  because  of 
the  great  limpidity  of  the  atmosphere  in  these  regions. 

The  zodiacal  light,  when  it  is  distinctly  seen,  as  in  the  intertropical 
zone,  is  one  of  the  most  beautiful  of  the  celestial  phenomena.  Its  color 
is  pure  white.  Certain  observers  in  Europe  have  sometimes  thought 
that  they  could  discern  a  reddish  tint  in  it.  This  tint  has  no  real  exist- 


THE  ZODIACAL  LIGHT.  175 

ence ;  for,  if  it  had,  it  would  be  most  distinctly  discerned  at  the  tropics, 
as  the  color  would  become  more  perceptible  when  the  intensity  of  the 
light  was  increased.  The  last  traces  of  twilight  have  been  mistaken  for 
it.  In  the  tropics  (in  the  months  of  January  and  February,  for  the 
Tropic  of  Cancer)  it  rises  perpendicularly  to  the  horizon ;  then,  when 
night  has  fully  set  in,  there  is  seen  rising  in  the  west  a  beautiful  white 
vertical  column,  the  central  axis  of  which  equals  and  even  exceeds  in 
intensity  the  more  brilliant  parts  of  the  Milky  Way.  Upon  the  edges 
of  this  column,  the  light  gradually  blends  with  the  feeble  glimmer  of 
the  sky.  It  differs  in  that  respect  from  the  Milky  Way,  the  edges  of 
which  at  certain  points  offer  a  noticeable  contrast  of  light  to  the  general 
darkness,  as  in  the  black  hollow  of  the  Southern  Cross,  called  the  coal- 
sack. 

It  is  not  visible  in  Europe  during  the  summer.  This  is  owing  to  its 
inclined  position  upon  the  southern  horizon,  which  then  grazes  the  part 
of  the  zodiac  which  is  visible  at  night  and  during  the  twilights.  In 
February  its  appearance  is  most  complete.  In  warm  countries,  the 
shortness  of  twilights,  and  the  elevation  of  the  ecliptic,  cause  the  phe- 
nomenon to  be  visible  all  the  year  round.  There  are,  however,  even  in 
countries  where  this  is  the  case,  periodical  maxima  of  beauty  which  de- 
pend upon  the  inclination  of  the  plane  of  the  zodiac  to  the  horizon. 

The  observations  of  Cassini  and  of  Mairan,  who  sometimes  saw  the 
zodiacal  light  at  more  than  100°  from  the  sun,  had  long  since  indicated 
that  this  beautiful  phenomenon  extends  beyond  the  terrestrial  orbit. 
Humboldt  and  Brorsen  had  also  remarked  a  luminous  thread  uniting 
the  east  and  west. 

Let  us  now  consider  what  is  the  nature  of  this  nebulosity  which  sur- 
rounds the  sun.  Several  astronomers  of  the  last  century  thought  it  was 
the  atmosphere  of  that  luminary,  extending  to  an  immense  distance  in 
the  direction  of  its  equator.  From  mathematical  considerations,  La- 
place has  shown  that  this  hypothesis  is  inadmissible,  and  that  the  solar 
atmosphere  can  not  extend  beyond  the  limit  at  which  the  centrifugal 
force  due  to  rotation  would  be  in  equilibrium  with  the  attraction  of  the 
sun.  It  can  easily  be  shown  that  at  a  distance  from  the  sun  equal  to 
thirty-six  times  its  semi-diameter,  the  centrifugal  force  developed  by  its 
rotation  equals  the  weight  of  the  atmospheric  particles  at  that  distance. 
It  is  mathematically  impossible  that  the  solar  atmosphere  can  extend 
beyond  this  limit.  It  is  not  half  the  distance  from  Mercury  to  the  sun, 
and  but  a  sixth  part  of  the  distance  at  which  the  earth  gravitates,  for  we 


IfQ  THE  ATMOSPHERE. 

are  situated  at  a  distance  of  two  hundred  and  fourteen  times  the  semi- 
diameter  of  this  gigantic  luminary  from  its  centre.  Therefore  the  zodi- 
acal light,  which  extends  beyond  the  terrestrial  orbit,  is  not  an  atmos- 
phere of  the  sun. 

Physicists  have  ascertained  that  all  reflected  lights  acquire  the  prop- 
erties peculiar  to  polarization,  but  that  at  the  same  time  these  proper-, 
ties  may  be  lost  in  the  event  of  the  reflection  arising,  not  from  a  gas  or 
a  continuous  surface,  but  from  a  series  of  distinct  particles,  as  in  the 
clouds,  which  are  composed  of  globules  of  water.  The  zodiacal  light 
not  being  polarized,  it  results  either  that  this  light  is  not  reflected,  and 
issues  directly  from  matter  luminous  in  itself,  or,  if  it  proceeds  from  the 
sun,  that  it  is  caused  by  the  reflection  of  the  light  of  that  luminary  from 
a  multitude  of  corpuscles  having  no  connection  with  each  other,  but  obe- 
dient, like  all  matter,  to  the  laws  of  universal  gravitation.  These  bodies 
we  must  regard  as  circulating  round  the  sun,  and  describing  elliptical 
orbits  like  the  planets  or  the  comets.  Now,  if  the  zodiacal  light  pro- 
ceeded from  matter  luminous  in  itself,  this  substance  would  still  reflect 
a  certain  quantity  of  the  solar  light,  so  that  traces  of  polarization  in  the 
zodiacal  light  would  be  perceived  if  it  was  not  composed  of  distinct 
corpuscles.  Therefore,  in  any  case,  we  may  consider  as  proved  that  it 
is  due  to  corpuscles  with  no  connection  between  each  other,  and  cir- 
culating in  accordance  to  the  laws  of  gravitation  round  the  sun,  from 
which  they  receive  their  light.  Judging  by  the  trifling  intensity  of 
the  light  which  they  shed,  it  is  improbable  that  they  further  possess  a 
proper  light  of  their  own. 

It  is  possible  that  the  aerolites,  to  the  number  of  milliards  upon  mil- 
liards, distributed  throughout  the  whole  planetary  system,  and  chiefly 
in  the  general  plane  of  movement — that  is,  in  the  plane  of  the  ecliptic 
—  the  bolides,  the  shooting-stars,  corpuscles,  solid,  liquid,  and  gaseous, 
form  but  one  general  kind  of  celestial  fragmentary  bodies,  and  that  the 
zone  in  which  they  chiefly  gravitate  is  manifested  to  us  by  the  reflec- 
tion of  the  solar  light,  and  constitutes  the  zodiacal  light ;  and  that,  by 
falling  against  the  sun,  these  corpuscles  cause  the  spots  on  its  disk,  and 
help  to  keep  up  its  immense  heat.  If  this  whirlwind  of  corpuscles 
does  not  circulate  around  the  sun  itself — a  fact  not  proved  —  it  circu- 
lates around  the  earth;  and  it  is  just  possible  that  from  afar  it  may 
look  like  the  ring  of  Saturn. 

The  appearance  of  the  zodiacal  light  is  somewhat  rare  in  France ;  it 
is  scarcely  ever  seen  distinctly  more  than  once  or  twice  a  year,  and  then 


THE  ZODIACAL  LIGHT.  177 

in  February.  It  was  seen  in  Paris  very  clearly  on  the  20th  of  Febru- 
ary, 1871,  and  lasted  from  6'50  to  7'30.  In  the  shape  of  a  spindle,  in 
which  it  is  always  seen,  it  measured  18°  in  width  at  its  base,  at  the  ho- 
rizon, and,  rising  obliquely  along  the  zodiac,  terminated  in  a  point  be- 
fore reaching  the  Pleiades.  From  the  sun,  which  had  set  an  hour  and 
a  half  earlier,  to  the  extremity  of  the  spindle,  it  measured  86°;  the  part 
which  was  visible  above  the  horizon  measured  63°. 

The  determination  of  its  intensity  was  all  the  more  easy,  as  the  at- 
mosphere of  Paris  was  scarcely  lighted  up  at  all,  in  consequence  of 
there  being  no  gas.  Calm  and  motionless,  this  light  was  very  different 
from  the  quivering  gleam  of  the  aurora  borealis.  This  spindle  was 
much  more  intense  in  the  middle  than  at  the  edges,  and  at  its  base  than 
at  its  apex.  The  tint,  about  half  as  brilliant  again  as  the  Milky  Way, 
was  rather  more  yellow.  The  smallest  stars  were  visible  through  this 
veil;  while  in  the  case  of  the  aurora  borealis  in  October,  1870,  the  brill- 
iancy of  the  stars  in  Ursa  Major  was  eclipsed. 

12 


BOOK  THIRD, 

TEMPERATURE. 


HEAT. 


CHAPTER  I. 

HEAT:  THE  THERMOMETER — QUANTITY  OF  HEAT  RECEIVED — TEMPER- 
ATURE OF  THE  SUN — TEMPERATURE  OF  SPACE. 

WE  have,  in  the  First  Book,  contemplated  the  earth  as  it  is  borne 
along  in  the  midst  of  space  by  the  force  of  universal  gravitation,  revolv- 
ing in  an  orbit  distant  91£  millions  of  miles  from  the  sun,  which  not 
only  retains  it,  but  also  gives  it  beauty  and  life.  From  it  we  also  de- 
rive heat,  to  the  consideration  of  which  we  now  proceed.  Let  us  first 
see  how  heat,  and  its  distribution  over  the  surface  of  the  globe,  are  to 
be  estimated. 

To  measure  the  variations  of  temperature,  the  thermometer  (Oepfiog, 
heat;  /utrpov,  measure)  is  used,  just  as  the  barometer  was  invented,  as 
we  have  seen  above,  for  ascertaining  the  variations  in  atmospheric 
pressure.  Without  discussing  at  greater  length  the  employment  of  the 
thermometer,  or  the  various  forms  of  the  instrument,  than  we  did  the 
above  contrivance,  it  is,  nevertheless,  interesting  to  go  back  to  its  dis- 
covery, which  also  dates  from  the  middle  of  the  seventeenth  century. 

Our  ancestors  judged  of  temperature  pretty  much  in  the  same  way  as 
we  do  in  the  present  day,  viz.,  by  the  principal  effects  resulting  from  it. 
Nowadays,  science  measures  it  more  in  detail  and  more  uniformly  by 
means  of  special  instruments  which  permit  of  a  comparison  between  the 
results  obtained  in  different  countries,  or  between  those  of  one  epoch 
and  another.  When  the  academicians  of  Florence  established  the  fact 
that  all  bodies  undergo  a  change  in  volume  under  the  influence  of  heat, 
they  laid  the  basis  of  thermometry.  The  instrument  of  which  these  sa- 
vans  made  use  consisted  of  a  sphere  soldered  to  a  narrow  tube,  and  con- 
taining colored  alcohol.  When  this  apparatus  is  transferred  from  one 
place  to  another  warmer  place,  the  liquid  becomes  dilated  and  the  level 
rises,  thus  showing  the  augmentation  of  the  temperature.  This  appara- 
tus dates  from  1660.  In  order  that  thermometers  might  be  suitable  for 
comparing  with  each  other  (that  they  might,  that  is  to  say,  give  the 
same  indications  under  the  same  circumstances),  the  academicians  of 
Florence  had  them  all  constructed,  as  nearly  as  was  possible,  upon  one 
standard.  A  natural  philosopher  of  Pavia,  one  Charles  Renaldi,  was 


}g2  THE  ATMOSPHERE. 

the  first  to  suggest,  about  1694,  the  means,  still  in  use,  for  obtaining 
thermometers  suitable  for  making  comparisons.  The  plan  consists  in 
placing  the  instrument  successively  in  two  calorific  positions,  invariable 
and  easy  of  reproduction,  viz.,  those  corresponding  to  the  melting  of  ice 
and  the  boiling  of  water.  Between  these  limits  of  temperature  any 
given  body  becomes  dilated  by  the  same  fraction  of  its  volume.  As 
a  rule,  0  is  marked  at  the  point  at  which  the  liquid  of  the  thermometer 
stands  in  melting  ice,  and  100  at  the  point  where  it  remains  stationary 
in  the  midst  of  boiling  water.  These  two  points  being  marked  upon 
the  stem,  the  interval  between  them  is  divided  into  one  hundred  equal 
parts.  Newton,  having  conclusively  demonstrated  the  fixity*  of  the 
point  at  which  water  boils,  the  means  adopted  by  Kenaldi  to  render 
thermometers  .capable  of  comparison  was  adopted  by  all  physical  phi- 
losophers. This  is  the  Centigrade  thermometer,  the  most  convenient, 
and  the  most  in  use.f  Thirty  years  ago,  Pouillet  engaged  in  a  series 
of  ingenious  and  patient  experiments,  with  a  view  of  determining  the 
quantity  of  heat  transmitted  to  the  earth  by  the  sun,  and  the  tempera- 
ture of  space — that  is  to  say,  the  two  constituent  elements  of  the  tem- 
perature of  the  globe. 

The  two  contrivances  made  use  of  for  the  purpose  were  the  pyrheli- 
ometer  and  the  actinometer.  The  latter,  being  only  used  for  researches 
as  to  the  temperature  of  the  zenith,  need  not  occupy  our  attention 
here. 

The  pyrheliometer  is  in  principle  composed  of  a  thin  silvered  box,  A 
(see  Fig.  47),  four  or  five  inches  in  diameter,  and  holding,  perhaps,  three 
or  four  ounces  of  water.  Its  surface,  turned  toward  the  sun,  is  black- 
ened. A  thermometer  is  introduced  into  the  box  and  embedded  in  the 
copper  frame- work,  B.  The  water  in  the  box,  at  the  same  temperature 
as  the  surrounding  air,  is  exposed  for  five  minutes  to  the  sun.  In  order 
to  ascertain  that  the  side  of  the  box  is  quite  perpendicular  to  the  sun's 
rays,  care  is  taken  to  see  that  its  shadow  falls  exactly  upon  the  lower 
disk,  c,  of  the  same  diameter.  By  comparing  its  temperature  with  the 
temperature  of  the  air  previous  and  subsequent  to  its  exposure,  the 

[*  That  is  to  say,  under  the  same  atmospheric  pressure.  The  boiling-point  of  water  varies 
every  day  with  the  height  of  the  barometer ;  and,  in  fact,  a  method  often  used  by  explorers  for 
determining  the  height  of  the  barometer  (and  therefore  their  own  elevation  above  the  sea-level) 
is  to  find  the  temperature  at  which  water  boils  at  the  place  in  question. — ED.] 

[t  The  thermometer  used  in  England  is  Fahrenheit's.  The  temperature  of  melting  ice  is 
marked  32°,  and  that  of  boiling  water  (when  the  height  of  the  barometer  is  29 -92  in.)  212° ; 
thus  180  graduations  on  the  Fahrenheit  scale  correspond  to  100  on  the  Centigrade.— ED.] 


HEAT.  183 

quantity  of  heat  received  from  the  sun  in  a  minute  by  each  square  inch 
of  ground  can  be  found  and  expressed  in  heat-units.* 

Making  allowance  for  the  atmospheric  strata  traversed  by  the  solar 
rays,  the  experimentalist  discovered  that  the  pyrheliometer  would  be 
raised  12°  1  Fahr.  if  the  atmosphere  were  capable  of  transmitting  in  its 
totality  all  the  solar  heat,  without  itself  absorbing  any,  or  if  the  appa- 
ratus could  be  placed  at  the  limits  of  the  atmosphere  to  receive  at  that 
point,  without  any  loss,  the  heat  transmitted  to  us  by  the  sun. 


Fig.  47.— The  Pyrheliometer. 


We  can  thus  tell  the  quantity  of  heat  which  the  sun  spreads  in  the 
space  of  a  minute  over  a  square  inch  at  the  limit  of  the  atmosphere,  and 
which  would  also  be  received  at  the  surface  of  the  ground,  were  it  not 
that  the  air  of  the  atmosphere  absorbed  some  of  the  rays  as  they  passed 
through  it. 

From  these  data  and  the  law  in  accordance  with  which  transmitted 

[*  The  heat-unit  generally  adopted  in  English  works  is  the  quantity  of  heat  necessary  to 
raise  one  pound  of  ice-cold  water  one  degree  Centigrade,  viz.,  to  raise  one  pound  of  water  from 
0°  to  1°  C.— ED.] 


134  THE  ATMOSPHERE. 

heat  diminishes  in  proportion  as  the  obliquity  increases,  it  is  easy  to 
calculate  the  proportion  of  incident  heat  which  arrives  each  instant  upon 
the  lighted  hemisphere  of  the  globe,  and  the  proportion  absorbed  in  the 
corresponding  half  of  the  atmosphere.  The  calculation  shows  that 
when  the  atmosphere  is,  to  all  appearance,  quite  still,  it  is  absorbing 
nearly  one-half  of  the  total  quantity  of  heat  which  the  sun  emits  to  us, 
and  that  it  is  only  the  other  half  of  this  heat  which  reaches  the  ground. 
Since  the  sun,  as  has  been  calculated,  transmits  every  minute  to  each 
square  yard  of  the  ground  that  it  shines  perpendicularly  upon  a  de- 
gree of  heat  equal  to  about  35,200  heat-units,  it  is  easy  to  conclude 
therefrom  the  total  quantity  of  heat  which  the  terrestrial  globe  and 
its  atmosphere  together  receive  in  a  year.  The  result  is  more  than 
2,660,000,000,000,000,000,000  heat -units!  This  heat  would  raise,  if 
such  were  possible,  by  2315  degrees,  a  body  of  water  three  feet  three 
inches  deep,  and  enveloping  the  whole.  By  transforming  this  quantity 
of  heat  into  a  quantity  of  melted  ice,  the  following  result  is  arrived  at : 
If  the  total  quantity  of  heat  which  the  earth  receives  from  the  sun  in 
the  course  of  the  year  was  uniformly  distributed  over  all  parts  of  the 
globe,  without  any  loss  in  melting  ice,  it  would  be  sufficient  to  melt  a 
coat  of  ice  enveloping  the  whole  globe  to  a  depth  of  about  one  hundred 
feet.  Such  is  the  simplest  way  of  expressing  the  total  quantity  of  heat 
which  the  earth  receives  each  year  from  the  sun. 

It  is  this  gigantic  quantity  of  heat  which  sets  in  motion  the  mechan- 
ism of  terrestrial  action,  which  lets  loose  the  tempests  over  the  ocean, 
and,  in  a  word,  sustains  the  vast  aerial  life  of  this  planet.  The  same 
fundamental  data  permit  of  our  ascertaining  the  total  amount  of  heat 
which  is  emitted  from  the  sun  in  a  given  time. 

Let  us  consider  this  luminary  as  the  centre  of  a  vast  sphere,  the  ra- 
dius of  which  is  equal  to  the  mean  distance  of  the  earth  from  it;  then  it 
is  evident  that  over  the  surface  of  this  sphere  each  square  yard  receives 
every  minute  from  the  sun  precisely  as  much  heat  as  the  square  yard 
of  the  earth— that  is  to  say,  35,200.  Consequently,  the  total  quantity  of 
heat  which  it  receives  is  equal  to  its  entire  surface,  expressed  in  yards 
and  multiplied  by  35,200. 

The  same  thing  may  be  expressed  by  stating  that  the  terrestrial 
globe,  with  its  8000  miles  of  diameter,  only  intercepts,  in  this  sphere  of 
91£  million  miles  of  radius,  aaooo^oooo  °f  the  total  heat  that  leaves  that 
luminary,  and  that  the  heat  emitted  by  the  sun  is  2,300,000,000  greater 
than  that  received  by  the  earth. 


HEAT.  185 

Transforming  into  the  quantity  of  melting  ice,  we  obtain  the  follow- 
ing result : 

If  the  total  amount  of  heat  emitted  by  the  sun  were  exclusively  em- 
ployed in  melting  a  coat  of  ice  placed  right  around  the  body  of  the  sun, 
it  would  be  capable  of  melting  in  a  minute  a  thickness  of  nearly  39f 
feet — that  is,  a  thickness  of  more  than  lOf  miles  in  the  twenty-four 
hours ! 

One  part  of  this  immense  source  of  "  energy  is  employed  in  heating 
the  terrestrial  rind  to  a  certain  depth ;  but  as  the  soil  and  the  atmos- 
phere radiate  into  space,  and  as  the  terrestrial  globe  does  not  seem  to 
lose  or  gain  in  reference  to  the  mean  temperature,  at  least  during  long 
periods  of  years,  all  this  part  of  the  sun's  radiation  may  be  considered 
as  maintaining  the  equilibrium  of  the  temperature  of  our  planet.  An- 
other part  is  transformed  into  molecular  movements,  in  chemical  action 
and  reactions,  which  are  the  source  whence  the  life  of  animals  and  veg- 
etables derives  unceasingly  the  wherewithal  of  their  perpetuation  and 
sustenance.  Heat,  which  thus  seems  necessary  for  these  beings,  is  but 
an  emanation  from  our  luminary.  As  Tyndall  remarks,  "It  is  thus 
we  are,  not  merely  in  the  poetical  sense,  but  practically,  children  of  the 
sun." 

The  American  engineer,  Ericsson,  the  inventor  of  the  solar  steam-en- 
gine, has  calculated  that  the  mechanical  effect  of  the  solar  heat  which 
falls  upon  the  roofs  at  Philadelphia  would  keep  in  motion  more  than 
5000  steam-engines,  each  of  20-horse  power. 

The  work  done  in  raising  the  temperature  of  a  pound  of  water  by 
one  degree  Fahrenheit  is  exactly  as  great  as  that  required  to  raise  a 
weight  of  one  pound  to  the  height  of  772  feet. 

Solar  heat  is  the  source  of  the  only  natural  works  that  man  has  yet 
been  able  to  divert  to  his  profit,  and  among  them  we  must  include 
water-courses  and  the  winds. 

Moreover,  the  combustible  matter  of  manufacture  is  derived  from  the 
same  luminary ;  as  wood,  it  is  carbon  absorbed  by  the  vegetables  breath- 
ing in  the  air  under  the  influence  of  the  sun  ;  as  coal,  it  is  still  carbon 
that  has  been  in  earlier  ages  fixed  by  the  same  influence  in  the  large 
antediluvian  forests. 

The  sun's  rays,  after  having  traversed  either  the  air,  a  pane  of  glass, 
or  any  transparent  body,  lose  the  faculty  of  retreating  through  the  same 
transparent  body  to  return  toward  celestial  space.  It  is  by  a  procedure 
founded  upon  this  physical  law  that  gardeners  accelerate  in  spring  the 


IQQ  THE  ATMOSPHERE. 

vegetation  of  delicate  plants  by  covering  them  with  a  glass  bell,  admit- 
ting the  solar  rays,  which  have  great  difficulty  in  effecting  their  egress. 
If  the  gardener  places  two  or  three  of  these  bell-glasses  one  upon  an- 
other, he  invariably  burns  up  the  plant  underneath  them,  and  even  in 
the  mild  weather  of  March,  or  April  he  is  often  obliged  to  raise  one  of 
the  edges  of  the  glass  to  prevent  the  plant  from  being  injured  by  the 
sun  at  noon.  By  means  of  an  apparatus  composed  of  a  box  blackened 
inside,  and  of  several  pieces  of  glass  laid  one  upon  the  other,  Saussure 
was  enabled  to  raise  water  to  boiling-point;  and  Sir  John  Herschel, 
during  his  stay  at  the  Cape  of  Good  Hope,  in  the  burning  heat  of  the 
last  days  of  December,  was  enabled  to  cook  a  piece  of  "  bceuf  a  la  mode" 
of  very  fair  dimensions,  by  means  of  two  blackened  boxes  placed  one 
inside  the  other,  and  each  provided  with  one  single  glass,  with  no  other 
source  of  heat  than  the  solar  rays  which  were  ingulfed  without  possi- 
bility of  escape  in  this  kind  of  trap.  "  There  was,"  says  M.  Babinet, 
"sufficient  to  regale  the  whole  of  his  family  and  their  guests  at  this 
meal,  prepared  with  a  stove  of  such  a  novel  kind." 

Herschel's  box,  closed  only  by  two  panes  of  glass,  reached  successive- 
ly 80, 100,  and  120  degrees  of  heat. 

Although  this  oven  appears  so  novel,  it  may  almost  be  said  to  be 
taken  from  the  Greeks.  We  find,  indeed,  that  a  century  before  the 
Christian  era,  Hero  of  Alexandria  described  in  his  "Pneumatics"  a 
large  number  of  ingenious  contrivances  devised  by  the"  ancients,  and,  no 
doubt,  by  the  learned  hierophants  of  Egypt.  One  of  these,  which  seems 
to  have  been  constructed  by  Hero,  draws  water  from  a  reservoir  by  the 
sole  effect  of  the  dilatation  and  condensation  of  air  under  the  influence 
of  the  sun  alternately  shining  on  and  concealed  from  the  apparatus. 

At  the  close  of  the  sixteenth  century,  the  Neapolitan  savant,  J.  B. 
Porta,  set  forth  in  his  "Natural  Magic"  the  mechanical  applications  of 
solar  heat.  If,  he  says,  a  hollow  copper  globe  is  placed  upon  the  sum- 
mit of  a  tower,  and  if  from  it  there  descends  a  pipe  into  a  reservoir  of 
water,  by  heating  the  globe  above,  either  by  means  of  fire  or  the  sun's 
rays,  the  rarefied  air  escapes.  Soon  after,  when  the  sun  declines,  the 
copper  globe  cools,  the  air  becomes  condensed,  and  the  water  rises  up 
the  pipe. 

The  concentration  of  solar  heat  in  a  glass-covered  inclosure  is  an  ex- 
periment so  easy  that  the  observation  of  it  must  have  followed  very 
closely  upon  the  invention  of  glass.  Nevertheless,  despite  the  different 
proofs  of  this  fact  and  the  applications  of  its  principle  to  which  I  have 


HEAT.  187 

alluded,  there  is  no  complete  scientific  study  of  the  phenomenon  earlier 
than  that  of  Saussure.  Subsequent  to  his  work  and  that  of  Herschel 
the  subject  had  been  considered  in  various  lights  by  different  philoso- 
phers. This  curious  problem  is  just  now  in  perhaps  its  most  interest- 
ing phase,  viz.,  that  which  gives  on  the  one  hand  serious  results,  and 
on  the  other  allows  the  imagination  to  guess  at  others  in  the  future  still 
more  important. 

It  is  a  natural  question  to  ask,  What  is  the  temperature  of  the  sun? 
To  this  we  can  give  no  satisfactory  answer.  Two  estimates  have  been 
made  by  Secchi  and  Zollner,  which,  however,  differ  enormously,  the 
former  giving  about  19,000,000°  Fahr.,  while  the  latter  only  amounts  to 
about  49,000°  Fahr. 

To  determine  the  temperature  of  the  sun,  an  apparatus  has  been  used 
which  exposes  the  thermometer  to  its  rays  in  an  inclosed  place,  the 
temperature  of  which  is  previously  ascertained.  Eeading  the  indica- 
tion given  by  the  mercurial  column,  the  number  is  multiplied  by  the 
ratio  of  the  surface  of  the  celestial  sphere  to  the  apparent  surface  of  the 
sun.  As  the  solar  disk  has  a  mean  diameter  of  31'  3" '6,  the  ratio  of 
the  whole  celestial  sphere  to  this  is  183,960.  The  apparatus  in  ques- 
tion is  as  follows:  Two  concentric  cylinders  soldered  together  form  a 
kind  of  double  caldron,  the  annular  interval  of  which  may  be  filled 
with  water  or  oil  at  a  given  temperature.  A  thermometer  passes  by 
means  of  a  small  tube  through  the  annular  space  and  penetrates  to  the 
axis  of  the  cylinder,  where  it  receives  the  solar  rays,  which  are  intro- 
duced by  means  of  a  diaphragm,  the  orifice  of  which  is  scarcely  larger 
than  the  ball  of  the  thermometer.  The  interior  cylinder  and  its  ther- 
mometer are  covered  with  lamp-black ;  a  second  thermometer  gives  the 
temperature  of  the  annular  space  and,  consequently,  that  of  the  inclos- 
ure.  The  whole  apparatus  is  mounted  upon  a  stand  having  a  parallac- 
tic  movement,  corresponding  to  the  diurnal  motion  of  the  sun. 

The  apparatus  being  exposed  to  its  rays,  the  two  thermometers  are 
noticed,  the  difference  of  their  temperature  gradually  increases,  and  at 
the  end  of  a  certain  time  becomes  constant.  The  two  temperatures  are 
then  marked  and  the  difference  calculated. 

One  word  must  be  added  as  to  the  interior  heat  of  the  earth.  Mai- 
ran,  Buffon,  and  Bailly  estimated,  so  far  as  France  is  concerned,  the 
heat  which  escapes  from  the  interior  of  the  earth  at  twenty-nine  times 
as  much  in  summer,  and  four  hundred  times  as  much  in  winter,  as  that 
which  reaches  us  from  the  sun.  Thus,  the  heat  of  the  luminary  which 


jgg  THE  ATMOSPHERE. 

gives  us  light  would,  if  this  were  true,  form  but  a  small  fraction  of  that 
of  the  globe.  This  idea  was  developed  with  great  eloquence  in  the 
"  fipoques  de  la  Nature,"  but  the  ingenious  romance  to  which  it  forms 
a  basis  is  dispelled  like  a  phantom  before  the  stern  evidence  of  math- 
ematical calculations.  Fourier  having  discovered  that  the  excess  of  the 
temperature  of  the  terrestrial  surface  over  that  which  results  from  the 
mere  action  of  the  solar  rays  has  a  necessary  relation  to  the  increase  of 
the  temperatures  at  different  depths,  succeeded  in  deducing  from  the 
amount  of  this  increase,  as  found  by  experiment,  a  numerical  determi- 
nation for  the  excess  in  question — that  is,  for  the  thermometrical  effect 
which  the  central  heat  produces  upon  the  surface.  And,  instead  of  the 
high  figures  given  by  Mairan,  Bailly,  and  Buffon,  he  obtained  as  his  re- 
sult only  the  thirtieth  part  of  a  degree ! 

The  surface  of  the  globe,  which,  at  the  beginning  of  the  world,  was 
probably  incandescent,  has  cooled  down,  in  the  lapse  of  ages,  so  much 
as  to  retain  scarcely  a  trace  of  its  primitive  temperature.  Nevertheless, 
we  know  that  the  temperature  increases  as  we  descend  into  the  interior 
of  the  earth  at  the  rate  of  1°  to  about  112  feet,  on  the  average,  and  that 
the  heat  must  be  very  great  underneath  volcanoes.  Upon  the  surface 
(and  the  phenomena  of  the  surface  can  alone  alter  or  compromise  the 
existence  of  human  beings)  all  changes  are  limited  to  about  the  thir- 
tieth part  of  a  degree.  The  terrible  congelation  of  the  globe,  which 
Buffon  fixed  for  the  epoch  when  the  interior  heat  should  be  entirely 
dissipated,  is  therefore  a  mere  dream.* 

*  [M.  Flammarion  concludes  this  chapter  with  a  discussion  of  the  temperature  of  space,  and 
he  states  that  the  mechanical  theory  of  heat  shows  that  there  is  an  absolute  zero  of  temperature 
at  —459°  Fahr.  (—273°  Cent,),  so  that  no  body  can  be  colder  than  this;  it  being,  in  fact,  the 
temperature  of  a  body  totally  devoid  of  heat,  and  therefore  the  temperature  of  space.  I  should 
merely  have  contented  myself  with  the  omission  of  this  portion  of  the  chapter  without  remark, 
only  it  appears  to  me  that  the  eiyor  reproduced  by  M.  Flammarion  is  sufficiently  wide-spread  to 
make  it  worth  while  to  call  attention  to  the  matter.  In  point  of  fact,  we  have  no  evidence  for 
asserting  that  the  temperature  of  space  is  —459°.  We  know  that  gases,  at  ordinary  tempera- 
ture, expand  equally  by  heat, "so  that  if  a  thermometer  were  made  in  which  the  fluid  was  air 
kept  at  a  constant  pressure,  its  reading  would  be  the  same  as  if  any  other  gas  were  used,  the 
pressure  being  the  same.  Consider,  therefore,  a  thermometer  composed  of  air  contained  in  a 
long  straight  tube,  so  arranged  that  the  pressure  of  the  gas  is  kept  constant  whatever  its  vol- 
ume may  be,  and  suppose  the  freezing  and  boiling  points  determined  as  usual,  and  the  inter- 
vening space  divided  into  180  equal  parts,  as  in  Fahrenheit's  scale,  then  it  follows,  assuming 
Boyle  and  Mariotte's  law,  that  if  the  graduations  were  continued  right  down  to  the  end  of  the 
tube,  the  last  division  would  be  marked  very  nearly  —459°,  so  that  it  is  clear  that  no  tempera- 
ture, however  low,  can  correspond  to  — 459°  of  the  thermometer  (i.  e.,  the  air  thermometer  can 
never  read  so  low  as  — 459°),  as  in  that  case  the  air  would  have  been  compressed  into  nothing ; 


HEAT.  189 

but  as  it  is  clearly  convenient  to  start  from  the  end  of  the  tube,  this  point  can  very  well  be 
taken  as  our  zero,  merely  to  measure  from.  There  are  other  reasons,  of  a  more  strictly  scien- 
tific character,  derived  from  thermo-dynamical  considerations,  that  also  lead  to  approximately 
the  same  point  as  the  absolute  zero  of  temperature ;  but  they  do  not,  in. the  very  slightest  de- 
gree, imply  that  this  is  the  temperature  of  space ;  in  fact,  such  an  assertion  would  be  unintel- 
ligible, even  if  true,  without  much  explanation.  The  lowest  artificial  temperature  observed  is 
—220°  Fahr.  (—140°  Cent.),  obtained  by  Natterer,  by  exposing  to  evaporation  a  mixture  of 
nitrous  oxide  and  carbonic  disulphide. — ED.] 


THE  ATMOSPHERE. 


CHAPTER  II. 

HEAT  IN  THE  ATMOSPHERE. 

IT  now  becomes  necessary  to  ascertain  what  part  of  the  immense  cal- 
orific radiation  which  is  incessantly  emanating  from  the  sun  is  at  work 
in  the  atmosphere. 

Meteorology  is  nothing  but  a  great  physical  problem.  We  have  to 
determine  what  are  the  laws  which  regulate  the  manner  in  which  heat, 
barometrical  pressure,  vapor  of  water,  and  electricity,  are  distributed  in 
our  atmosphere,  in  relation  to  the  movements  which  the  solar  heat  en- 
genders in  the  solid,  liquid,  and  gaseous  superficial  stratum  of  our 
globe.  This  problem,  vast  as  it  is,  says  Father  Secchi,  is  in  reality  but 
an  application  of  the  best  known  laws  of  physics ;  the  difficulties  of 
solving  it  are  owing  rather  to  the  large  number  of  disturbing  causes, 
and  to  the  incalculable  reactions  of  effects  upon  causes,  than  to  any  real 
deficiency  in  the  general  theory.  Hence  the  necessity  of  numerous  ex- 
perimental data  in  order  to  arrive  at  a  complete  solution. 

The  atmosphere  is  in  reality  an  immense  machine,  to  the  action  of 
which  is  subordinated  every  thing  upon  our  planet  that  has  life. 
Though  there  are  neither  fly-wheels  nor  pistons  in  this  machine,  it  none 
the  less  does  the  work  of  millions  of  horses — a  work  the  aim  and  effect 
of  which  is  the  sustenance  of  life. 

All  the  movements  of  the  atmosphere  are  the  consequence  of  the 
property  which  gases  possess  of  being  expanded  by  heat.  The  varia- 
tions of  volume,  and,  consequently,  of  density,  are,  at  each  instant,  dis- 
turbing the  equilibrium  which  would  be  tending  to  establish  itself  in 
the  atmosphere.  The  air,  heated  in  the  equatorial  zones,  rises  into  the 
upper  regions  to  fall  again  near  the  poles ;  there  it  becomes  cool,  re- 
turns to  the  equator,  and  recommences  its  circulating  movement.  The 
work  thus  performed  in  the  atmosphere  is  enormous.  To  this  property 
of  gas  must  be  added  another,  not  less  important — that  of  dissolving* 

[*  The  air  and  the  vapor  of  water  form,  as  it  were,  different  atmospheres :  that  is  to  say, 
that  the  vapor  atmosphere  could  still  remain  if  all  the  air  were  removed.  Water,  placed  un- 
der the  air-pump,  evaporates  till  the  space  under  the  receiver  is  filled  with  aqueous  vapor  to  an 
extent  dependent  on  the  temperature.— ED.] 


HEAT  IN  THE  ATMOSPHERE.  191 

the  vapor  of  water  which,  as  it  rises  in  prodigious  quantities  about  the 
equator,  is  thence  distributed  all  over  the  earth  in  the  shape  of  rain. 
Thus  is  effected  another  and  scarcely  less  potent  work — the  distribution 
of  rain  over  the  surface  of  the  globe.  The  running  waters  which  set 
our  machines  in  motion  were  originally  raised  into  the  air  by  this 
mighty  agency ;  from  thence  they  pour  down  on  the  mountains  in  the 
shape  of  rain,  run  into  the  rivers,  and  so  make  their  way  again  into  the 
ocean  from  whence  they  started. 

The  sun  is  the  power  that  regulates  all  the  movements  of  the 
planetary  system ;  not  only  their  motions  in  their  orbits,  but  also 
the  physical  or  physiological  phenomena  which  take  place  upon  their 
surface.  On  the  earth,  in  particular,  the  atmospheric  movements  and 
those  of  the  waters,  the  development  of  vegetation,  the  production 
of  the  force  which  results  from  combustion  and  the  nutrition  of  ani- 
mals— all  these  phenomena  are  due  to  the  influence  of  the  sun's  heat 
rays. 

What  may  seem  to  us  still  more  perfectly  organized  is  the  way  in 
which  this  calorific  power  is,  so  to  speak,  stored  up  in  the  vegetables ; 
not  only  in  those  which,  still  alive,  serve  for  our  use  and  nourishment, 
but  also  in  those  which,  buried  for  ages  in  the  bowels  of  the  globe,  at 
length  emerge  therefrom  to  warm  us  and  supply  our  machines  with  the 
required  motive  power.  Each  plant  is  a  veritable  machine,  in  which 
are  elaborated  the  extremely  combustible  substances  which  serve  to 
furnish  us,  in  the  absence  of  the  sun,  with  heat  and  light,  or  to  produce, 
in  providing  us  with  nutriment,  the  force  and  vital  warmth  which  we 
stand  in  need  of.  It  is,  therefore,  on  the  sun,  as  Father  Secchi  again 
remarks,  that  depend  entirely  all  the  phenomena  of  nature,  and  our  ex- 
istence itself. 

In  the  solar  radiation  what  is  at  first  so  striking  is  the  light  which 
gives  us  day,  and  the  heat  which  warms  us ;  but,  besides  these  two  or- 
ders of  phenomena,  there  is  a  third  of  equal  importance,  viz.,  the  chem- 
ical actions  which  accompany  the  two  others.  Thus  three  classes  of  ac- 
tion must  be  distinguished  in  the  solar  work — the  luminous  rays,  the 
calorific  rays,  and  the  chemical  rays.  It  is  well  known  that,  to  analyze 
a  sunbeam,  it  is  passed  through  a  triangular  prism  of  glass,  on  emerging 
from  which  the  ray  is  decomposed  into  a  colored  ribbon,  as  we  have 
already  seen  in  our  study  of  the  rainbow.  But  the  visible  spectrum  is 
not  the  only  component  part  of  a  sunbeam.  The  many-hued  ribbon  is 
continued  at  each  end  by  an  invisible  ribbon.  The  waves — the  length 


1Q2  THE  ATMOSPHERE. 

of  which  is  included  between  -0000167  and  -0000266  of  an  inch— are 
capable  of  causing  our  optical  nerve  to  vibrate,  and  thus  producing  the 
sensation  of  light,  the  diversity  of  colors  being  dependent  only  upon  the 
length  of  the  waves,  the  longest  of  which  belong  to  the  red  rays,  and 
which  gradually  diminish  toward  the  violet  To  the  left  of  the  red  ex- 
tremity of  the  spectrum,  there  are  long  and  slow  waves  of  heat.  To 
the  right  of  the  violet  end,  there  are  short  and  rapid  waves  of  chemical 
action.  The  eye  sees  neither  the  first  nor  the  second  of  these,  but  they 
may  be  recognized  by  the  use  of  suitable  apparatus.  In  reality,  how- 
ever, there  exists  in  nature  but  one  single  series  of  waves,  the  lengths 
of  which  continually  decrease  from  the  extremity  of  the  obscure  calo- 
rific spectrum  to  the  extremity  of  the  invisible  chemical  spectrum. 
Between  these  two  extremes  there  is  but  a  very  limited  part  which  has 
the  power  of  giving  sensation  to  the  optical  nerve. 

Fig.  48  shows  the  relative  extent  and  intensity  of  these  different  ac- 
tions, separated  from  each  other  as  they  are  made  manifest  to  us  by  the 


Fig.  48.— Relative  Intensity  of  the  calorific,  luminous,  and  chemical  rays  of  the  sun. 

dispersive  action  of  a  prism.  The  band  which  forms  the  basis  of  this 
figure  indicates  the  length  of  the  solar  spectrum.  From  A  to  H  is  the 
luminous  part ;  to  the  right,  from  H  to  P,  is  the  invisible  chemical  part ; 
to  the  left,  from  A  to  s,  is  the  calorific  part,  also  invisible.  The  curves 
traced  above  show  the  relative  intensities  at  each  point  of  the  spectrum. 
The  intensity  of  the  light  is  represented  by  the  curve  R'  M'  T',  that  of 
chemical  action  by  m  M"  P,  that  of  calorific  radiations  by  R  M  T.  It  has 
been  attempted  to  represent  the  three  respective  intensities  by  the  three 
bands,  1  (light),  2  (heat),  and  3  (chemical  action). 

Thus  we  do  not  see  all  that  goes  on  in  nature.     The  luminous  rays 


HEAT  IN  THE  ATMOSPHERE.  193 

are  the  only  ones  which  we  can  behold ;  but  the  calorific  and  chemical 
rays  take  effect  without  being  visible  to  us. 

The  illuminating  power  of  the  different  rays  consists  in  their  greater 
or  less  capacity  for  giving  an  impulse  to  the  optical  nerve.  It  is  prob- 
able that  the  faculty  of  perceiving  luminous  phenomena  has  not  the 
same  scale  for  every  individual,  and  that  it  is  much  more  extended  in 
the  case  of  certain  animals  than  with  man,  both  at  the  red  and  violet 
ends  of  the  spectrum.  Pure  water  possesses  a  very  considerable  absorb- 
ing power  for  thermal  rays.  The  moisture  contained  in  the  eye  differs 
very  little  from  pure  water,  and  it  is  this  fact  which  very  likely  renders 
the  organ  of  sight  insensible  to  calorific  rays.  The  extent  of  the  lu- 
minous waves  which  are  sensible  to  the  eye  ordinarily  corresponds  to 
what  is  called  in  acoustics  an  octave,  so  that  man  is  only  placed  in  re- 
lation with  nature  by  a  very  small  part  of  the  solar  rays. 

Gases  possess  the  faculty  of  absorbing  heat  rays,  and  consequently 
our  atmosphere  absorbs  a  very  considerable  portion  of  the  rays  which 
are  transmitted  to  us.  The  longest  waves  are  those  which  are  most 
easily  absorbed ;  thus  a  large  number  of  the  less  refrangible  rays  which 
fall  upon  our  atmosphere  are  stopped,  and  do  not  reach  us  at  all. 

The  absorption  produced  by  the  simple  gases,  oxygen  and  nitrogen, 
is  extremely  small ;  but  this  does  not  hold  good  of  the  compound  gases 
existent  in  our  atmosphere,  such  as  carbonic  acid,  vapor  of  water,  am- 
monia, etc.  Professor  P.  M.  Garibaldi,*  of  Genoa,  has  proved  by  con- 
clusive experiments  that,  for  a  pressure  of  29'92  inches,  these  gases  have 
absorption-powers  represented  by  the  following  figures: 

Atmospheric  air 1 

Carbonic  acid 92 

Ammonia 546 

Vapor  of  water 7937 

A  quantity  of  vapor  of  water  capable  of  producing  a  pressure  of  0'4 
inch  exercises  an  absorption  a  hundred  times  greater  than  that  of  at- 
mospheric air.  Thus,  a  considerable  portion  of  the  dark  heat  rays  pro- 
ceeding from  the  sun  are  intercepted  by  the  vapor  of  water  contained  in 
the  air,  and  are  unable  to  reach  the  surface  of  the  earth. 

The  luminous  rays  have  been  separated  from  the  heat  rays  by  Pro- 
fessor Tyndall.  To  effect  this,  a  pencil  of  solar  rays  was  made  to  pass 

[*  Professor  Tyndall  has  made  an  elaborate  and  careful  series  of  experiments  on  the  ab- 
sorptive power  of  different  gasas.  He  concludes  that  on  an  average  day  the  water  present  in. 
the  air  absorbs  about  sixty  times  as  much  heat  as  the  air  itself. — ED.] 

13 


-L94-  THE  ATMOSPHERE. 

through  a  solution  of  iodine  in  carbonic  disulphide.  The  rays  become 
invisible  without  losing  their  calorific  power,  and  if  the  vessel  contain- 
ing the  solution  is  made  in  the  shape  of  a  convex  lens,*  the  temperature 
in  the  focus  of  the  lens  is  increased  to  such  an  extent  that  combustible 
bodies  may  take  fire  there.  Professor  Tyndall  on  one  occasion  placed 
his  eye  at  the  focus,  and  the  retina  received  no  luminous  influence. 
The  calorific  rays  were,  however,  so  powerful  that  a  sheet  of  metal  was 
immediately  made  red-hot  at  the  very  spot  where  the  eye  had  received 
no  impression.  The  ratio  of  luminous  radiations  to  obscure  radiations 
is  equal  to  ^j-  for  incandescent  platinum.  For  sunlight,  the  heat  which 
accompanies  the  luminous  portion  of  the  spectrum  is  only  -£-  of  that 
which  is  found  in  the  obscure  part. 

The  action  of  the  atmosphere  is  to  raise  the  temperature  of  the  earth, 
for  it  allows  the  calorific  rays  to  reach  the  earth,  and  then  prevents  them 
from  making  their  way  back  into  space.  The  greater  portion  of  heat 
rays  sent  out  from  the  earth  are  no  longer  able  to  traverse  the  atmos- 
phere, while  only  a  few  of  the  sun's  rays,  which  are  of  high  tempera- 
ture, are  stopped. 

Further,  the  nocturnal  radiation  is  considerably  diminished  by  the 
presence  of  the  atmosphere,  and  in  this  way  the  cooling  of  the  globe 
and  of  the  plants  which  it  nourishes  becomes  modified  and  diminished. 
The  vapor  of  water  acts  very  efficiently,  and  a  moist  stratum,  only  a 
few  yards  thick,  arrests  the  nocturnal  process  of  cooling  as  completely 
as  the  whole  atmosphere. 

But  the  most  striking  fact  in  connection  with  this  matter  is  the  great 
absorption  of  heat  which  accompanies  the  transformation  of  water  (or 
any  other  liquid)  into  vapor.  The  heat  so  absorbed  is  termed  latent 
heat,  from  its  not  being  spent  in  raising  the  temperature  of  the  vapor. 
Water  evaporates  in  large  quantities,  especially  in  the  equatorial  regions, 
and  thus  absorbs  a  large  quantity  of  heat  which  remains  latent.  As 
much  heat  is  necessary  to  vaporize  one  pound  of  water  (at  the  tempera- 
ture of  the  boiling-point)  as  to  increase  by  1°  (Centigrade)  the  heat  of 
537  pounds  of  water.  The  vapor  of  water  absorbs  this  enormous  pro- 
portion of  heat,  which  it,  however,  restores  in  its  entirety  when  it  re- 
turns to  the  liquid  state  as  rain.  This  heat  is  destined  to  be  transport- 
ed to  the  most  distant  latitudes,  and  to  establish  in  the  atmospheric  en- 

[*  The  solution  must  be  inclosed  in  a  lens  of  rock-salt,  a  substance  which  allows  heat  to 
pass  through  it  without  absorbing  scarcely  any;  it  is  therefore  termed  diathermanous,  an  ad- 
jective having  the  same  reference  to  heat  that  transparent  has  to  light.— ED.] 


HEAT  IN  THE  ATMOSPHERE.  195 

velop  which  surrounds  the  globe  an  equality  of  temperature  which 
would  not  otherwise  be  produced.  The  quantity  of  heat  which  thus 
passes  from  the  equator  to  the  poles  is  beyond  conception. 

Thus,  for  instance,  numerous  and  rather  exact  observations  have 
taught  us  that  in  the  equatorial  regions  evaporation  each  year  removes 
a  body  of  water  at  least  sixteen  feet  deep.  Let  us  suppose  that  in  the 
same  regions  there  is  an  annual  rain-fall  of  rather  over  six  feet,  there 
still  remains  a  quantity  of  water  represented  by  a  depth  of  nearly  ten 
feet,  and  which  must  pass,  in  the  form  of  vapor,  into  the  countries  nearer 
to  the  poles.  The  surface  over  which  the  evaporation  takes  place  may 
be  estimated  at  seventy  million  geographical  miles;  and,  starting  from 
this  datum,  it  will  be  seen  that  the  depth  of  ten  feet  represents  a  volume 
of  water  equal  to  twenty-five  thousand  billions  of  cubic  feet  (25  x  1016). 
This  enormous  mass  of  heat  passes  incognito,  so  to  speak,  from  the  equa- 
tor to  the  poles,  transported  by  the  action  of  the  vapor,  and  this  latter, 
as  it  becomes  transformed  into  water  and  ice,  sets  free  all  the  heat  which 
it  had  absorbed,  thus  contributing  to  make  milder  the  climate  of  these 
desolate  regions.  In  this  way  the  heat  is  distributed  in  the  atmosphere, 
and  thus  are  created  clouds  and  rain,  which  will  be  explained  below. 

The  thickness  of  the  strata  of  air  traversed  by  the  solar  rays  has  a 
notable  influence  upon  heat  and  light.  The  rays  do  not  fall  upon  the 
earth  perpendicularly,  but  obliquely,  and  the  loss  is  greater  the  more 
they  are  inclined  to  the  vertical. 

This  diminution  has  been  submitted  to  different  calculations :  the  two 
formulas  which  seem  to  be  most  in  harmony  are  those  of  Bouguer  and 
Laplace.  Making  use  of  them,  the  following  results  are  arrived  at,  as 
to  the  thickness  of  the  strata  of  air  for  the  different  heights  of  the  sun : 


Height  above 
the  Horizon. 

Zenith's 
Distance. 

Thickness  of  the 
Strata  of  Air. 

90  deg. 

Odeg. 

1-00 

70 

20 

1-06 

50 

40 

1-30 

30 

60 

1-99 

20 

70 

2-90 

15 

75 

3-80 

10 

80 

5-51 

5 

85 

10-21 

4 

86 

12-15 

3 

87 

14-87 

2 

88 

18-88 

1 

89 

25-13 

0 

90 

35-50 

Thus,  if  the  thickness  of  the  atmosphere  traversed  by  a  ray  of  the 


196 


THE  ATMOSPHERE. 


sun  at  the  zenith  be  represented  by  1,  the  thickness  traversed  by  the 
sun's  rays  at  the  horizon  is  more  than  thirty-five  times  greater.  This 
difference  is  much  larger  than  can  be  indicated  in  the  annexed  illustra- 
tion (Fig.  49).  The  first 
result  of  this  inequali- 
ty is,  that  the  sunlight 
becomes  feebler  in  pro- 
portion as  the  sun  sinks 
toward  the  horizon.  At 
the  zenith  and  in  the 

Fig  49,-S^uality  of  the  thickness  of  air  traversed  by  the  San,  ac-    higher    regions    of  the 
cording  to  its  position  above  the  horizon.  g^y  ^e  gun  jg  dazzling, 

and  no  human  eye  can  withstand  its  blaze.  At  sunrise  and  at  sunset  we 
are  able  to  fix  our  eyes  upon  its  reddened  disk  without  inconvenience. 
The  smaller  stars  do  not  become  visible  till  they  reach  a  certain  height, 
and  we  can  only  witness  the  rising  and  setting  of  those  of  the  first  mag- 
nitude. According  to  the  researches  of  Bouguer,  if  10,000  be  taken  to 
represent  the  luminous  intensity  of  the  sun  as  it  would  be  seen  from  a 
point  external  to  the  atmosphere,  its  intensity  at  the  different  altitudes 
above  the  horizon  may  be  thus  stated : 


At  50  degrees.. 


7624 

6613 

5474 

3149 

1201 

802 

454 

192 

467 


That  is  to  say,  that,  at  sunrise  and  sunset,  this  luminary  has  only 
of  its  apparent  brilliance  when  at  the  zenith,  and  -rgVo-  of  its  brill- 
iance when  at  its  midday  elevation  over  our  horizon  during  the  sum- 
mer solstice.  These  comparisons  are  made  on  the  supposition  that 
the  sky  is  clear,  and  consequently  vary  with  the  more  or  less  misty 
state  of  the  atmosphere.  Heat  varies,  like  light,  with  its  angle  of  inci- 
dence. The  most  accurate  observations  prove  that  the  atmosphere  ab- 
sorbs, of  vertical  rays,  -28  of  the  heat  which  falls  upon  its  surface,  and 
the  total  absorption  in  the  illuminated  hemisphere  is  about  equal  to 


HEAT  IN  THE  ATMOSPHERE. 


197 


three-fifths  of  the  incident  heat ;  the  transmitted  part  at  different  heights 
being  represented  as  follows : 


Height. 

Amount 

transmitted. 

At  the  zenith  

0'72 

'  70  degrees  

0-70 

'  50       " 

0'64 

'  30       " 

0'51 

'10       "      

0'16 

'     0        "      :  

o-oo 

As  was  remarked  above,  it  is  not  the  air  itself — that  is  to  say,  the 
mixture  formed  of  oxygen  and  nitrogen — which  absorbs  the  most  heat, 
but  the  vapor  of  water,  which  always  exists  in  the  air,  but  m  very  vary- 
ing proportions. 

The  luminous  rays  pass  almost  in  their  entirety,  and  reach  the  ground ; 
the  heat  rays  are,  on  the  contrary,  absorbed  to  a  large  extent.  If,  there- 
fore, the  atmosphere  prevents  a  great  part  of  the  solar  heat  from  reach- 
ing the  surface  of  our  globe,  it  makes  up  for  it  by  retaining  for  us  the 
part  that  we  do  receive.  Without  the  atmosphere  and  the  vapor  of 
water  contained  in  it,  since  the  radiation  of  the  soil  goes  on  almost  with- 
out obstacle  toward  the  interplanetary  space,  the  loss  would  be  enor- 
mous, as  indeed  is  the  case  in  the  higher  regions.  No  sooner  has  the 
sun  set  than  a"  rapid  coldness  succeeds  the  intense  heat  of  the  sun's  di- 
rect rays ;  in  a  word,  there  is  an  enormous  difference  between  the  max- 
ima and  minima  of  temperature,  either  daily  or  monthly.  This  oc- 
curs upon  the  lofty  plateaux  of  Thibet,  and  explains  the  severity  of 
the  winters,  and  the  decline  of  the  isothermal  lines  in  these  regions. 
Tyndall  says  very  truly :  "  The  suppression,  for  a  single  summer's  night, 
of  the  vapor  of  water  contained  in  the  atmosphere  over  England  (and 
the  proposition  holds  true  for  all  the  countries  in  similar  latitudes)  would 
be  accompanied  by  the  destruction  of  all  the  plants  which  are  killed  by 
frost.  In  the  desert  of  Sahara,  where  the  ground  is  fire  and  the  wind  a 
flame,  the  cold  at  night  is  often  very  difficult  to  support.  In  this  hot 
country  ice  is  seen  to  form  in  the  course  of  the  night." 

Moisture  is  not  distributed  in  equal  proportions  at  all  elevations  of 
the  atmosphere.  We  shall  see,  further  on,  that  it  decreases  in  amount 
beyond  a  certain  height.  Heat  traverses  air  the  more  easily,  the  less 
moisture  it  contains.  After  the  lower  regions  of  the  atmosphere  have 
been  passed,  and  (say)  an  altitude  of  6000  feet  attained,  it  is  impossible 
to  avoid  noticing  the  very  considerable  increase  in  the  heat  of  the  sun 


-j^gg  THE  ATMOSPHERE. 

relatively  to  the  temperature  of  the  surrounding  air.  This  fact  never 
struck  me  so  strongly  as  in  an  aeronautical  ascent  on  June  10,  1867,  on 
which  occasion  I  noted,  at  7  A.M.,  at  a  height  of  10,000  feet  above  the 
ground,  that  there  was,  for  half  an  hour,  a  difference  of  27°  Fahr.  be- 
tween the  temperatures  of  my  feet  and  head ;  or,  to  speak  more  accurate- 
ly, between  the  temperature  of  the  interior  of  the  car  (shade),  and  that 
of  the  exterior  (sun).  The  thermometer  marked  46°  Fahr.  in  the  shade, 
and  73°  Fahr.  in  the  sun.  While  our  feet  felt  the  effects  of  this  relative 
cold,  a  hot  sun  scorched  our  necks  and  faces,  and  those  parts  of  the 
body  directly  exposed  to  the  solar  radiation.  The  effect  of  this  heat  is, 
of  course,  further  augmented  by  the  absence  of  the  slightest  current  of 
air* 

In  a  subsequent  ascent,  I  experienced  at  the  same  time  the  remarka- 
ble difference  of  36°  Fahr.  between  the  temperature  in  the  shade  and 
that  in  the  sun,  at  an  altitude  of  13,500  feet. 

The  influence  of  altitude  upon  the  intensity  of  the  sun's  calorific  in- 
fluence at  points  nearly  vertically  above  one  another  has  recently  been 
studied  very  carefuHy  by  M.  Desains  and  a  colleague  at  the  Schweitzer- 
hoff,  Lucerne,  and  at  the  Eighi-Culm  Hotel,  about  1500  yards  above  the 
lake.  These  experiments  demonstrated  that  at  the  same  hour,  and  un- 
der equal  conditions,  the  solar  radiation  was  more  intense  upon  the 
summit  of  the  Kighi  than  at  Lucerne,  but  that  it  was  less  capable  of 
being  transmitted  through  water.  It  was  found  that  the  solar  rays  in 
their  passage — at  an  angle  of  about  seventy  degrees  with  the  vertical — 
through  the  stratum  of  air  comprised  between  the  level  of  the  Eighi 
and  that  of  Lucerne,  underwent  a  loss  of  about  seventeen  per  cent. 

This  shows  that  the -terrestrial  temperatures  depend  not  only  on  the 
quantity  of  heat  received  from  the  sun,  but  also  and  especially  upon  the 
absorbing  power  of  the  air  in  regard  to  the  rays  of  heat.  Such  also,  no 
doubt,  is  the  case  in  the  other  planets;  and  the  influence  of  the  atmos- 
pheres is  such  that,  in  spite  of  its  close  proximity  to  the  sun,  it  is  possi- 
ble that  Mercury  may  possess  a  much  lower  temperature  than  that  of 
the  earth,  and  the  surface  of  Jupiter  may  present  a  climate  far  warmer 
than  ours,  in  despite  of  its  greater  distance  from  the  sun. 

Eecent  spectroscopic  experiments  of  M.  Janssen  render  the  existence 
of  planetary  atmospheres  generally  similar  to  our  own  probable.  As- 

[*  My  experiences  at  high  elevations  were  quite  opposed  to  those  stated  in  the  text.  At 
great  heights  I  observed  no  difference  in  reading  between  a  thermometer  with  the  sun  shining 
full  on  its  bulb,  and  another  in  which  the  bulb  was  carefully  shaded.— ED.] 


HEAT  IN  THE  ATMOSPHERE.  199 

tronomical  observations  also  long  since  pointed  to  the  same  conclusion 
with  regard  to  some  of  the  planets. 

After  having  appreciated  the  action  of  solar  heat  throughout  the  at- 
mosphere and  upon  the  surface  of  the  globe,  to  which,  in  fact,  almost 
every  movement  that  takes  place  there  can  be  traced,  we  must  now 
complete  the  account  by  noticing  that  the  amount  of  this  heat  dimin- 
ishes as  we  ascend  higher  into  the  atmosphere.  We  have  seen  that  the 
pressure  of  the  air  diminishes  in  proportion  as  we  rise  higher  into  it. 
The  temperature  is  subject  to  an  analogous  decrease,  which  may  be  esti- 
mated, though  not  nearly  so  accurately  as  in  the  case  of  the  diminution 
of  the  atmospheric  pressure.  Corresponding  to  the  indications  of  the 
barometer,  the  following  are  those  given  by  the  thermometer: 

When  an  ascent  in  a  balloon  is  made  with  the  sky  cloudy,  the  tem- 
perature generally  declines  until  the  clouds  have  been  reached;  once 
above  them,  a  rise  of  several  degrees  always  takes  place,  but  the  tem- 
perature soon  begins  to  fall  again.  With  a  clear  sky,  the  initial  tem- 
perature is,  cceteris  paribus,  higher  than  in  the  preceding  case  by  a  quan- 
tity about  equal  to  the  rise  observed  after  emerging  from  the  clouds. 
The  diminution  of  heat  is  never  absolutely  regular,  as  strata  of  hot  air 
are  nearly  always  encountered  in  the  atmosphere,  sometimes  five  or  six 
in  succession,  at  very  great  elevations.  These  alternations,  and  this  va- 
riability of  the  sky,  do  not  prevent  the  manifestation  of  one  general 
fact — that  of  the  decrease  in  the  temperature  with  an  increase  of  eleva- 
tion. 

The  following  is  the  result  of  a  series  of  observations  upon  this  point 
which  I  have  made  in  the  course  of  my  various  ascents : 

The  decrease  in  the  temperature  of  the  air,  which  plays  so  important 
a  part  in  the  formation  of  the  clouds  and  in  the  elements  of  meteorolo- 
gy, is  far  from  following  a  regular  and  fixed  law.  It  varies  according 
to  the  hours,  the  seasons,  the  state  of  the  sky,  the  direction  of  the 
winds,  the  condition  of  the  vapor  of  water,  etc.,  etc.  It  is  only  after  a 
great  number  of  observations  that  it  is  possible  to  deduce  any  fixed 
rule,  several  secondary  causes  which  must  be  first  ascertained  and  elim- 
inated being  always  at  work. 

From  several  observations,  taken  in  very  dissimilar  conditions  (which 
are,  however,  less  unfavorable  than  those  under  which  observations  are 
taken  on  mountain  sides),  it  follows  that  the  decrease  in  the  tempera- 
ture of  the  air  differs,  in  the  first  instance,  according  to  whether  the  sky 
is  clear  or  cloudy,  being  more  rapid  in  the  first  case  than  in  the  second. 


200  THE  ATMOSPHERE. 

With  a  clear  sky,  the  mean  fall  in  the  temperature  has  been  found  to 
be  7°  Fahr.  for  the  first  1600  feet  from  the  surface  of  the  ground ;  13° 
at  3280  feet;  19°  at  4900  feet;  23°  at  6560  feet;  27°  at  8200  feet;  31° 
at  9840  feet ;  34°  at  12,500  feet— an  average  of  1°  Fahr.  per  340  feet. 

With  the  sky  cloudy,  the  fall  in  the  temperature  is  5£°  Fahr.  for  the 
first  1500  feet;  11°  at  3000  feet;  16°  at  4900  feet;  19°  at  6560  feet; 
29°  at  9840  feet;  32°  at  12,500  feet— of  average  of  1°  Fahr.  per  350 
feet. 

The  temperature  of  the  clouds  is  higher  than  that  of  the  air  immedi- 
ately above  or  beneath  them.  The  decrease  is  more  rapid  near  the  sur- 
face of  the  ground,  and  more  gradual  at  greater  elevations.  It  is  also 
more  rapid  in  the  evening  than  in  the  morning,  and  also  in  warmer 
than  in  colder  weather.  Eegions  hotter  or  colder  than  the  mean  tem- 
perature for  their  altitude  are  sometimes  met  with  in  the  atmosphere, 
crossing  it  like  aerial  rivers.  Notwithstanding  these  variations,  the 
general  law  enunciated  above  is  the  expression  of  the  true  state  of 
things.  The  difference  between  the  indications  of  the  thermometer  in 
the  shade  and  in  the  sun  augments  with  elevation.  Thus,  the  general 
result  of  these  aerial  ascents  tends  to  show  that  the  temperature  de- 
creases about  1°  Fahr.  for  an  elevation  of  345  feet.  The  result  of  the 
well-known  and  numerous  aerostatical  observations  taken  by  Glaisher 
differs  but  little  from  the  above.  The  ascents  of  mountains  have  fur- 
nished a  certain  number  of  important  data,  among  which  may  be 
quoted  the  following : 

Humboldt  found  that  the  decrease,  in  a  southern  atmosphere,  was  1° 
Fahr.  to  344  feet  in  the  mountains,  and  to  440  feet  upon  the  table-lands. 
A  series  of  places  in  Southern  India  gave  320  feet ;  in  the  north  of  Hin- 
doostan,  on  the  other  hand,  the  decrease  was  1°  in  410,  an  amount  ap- 
proaching to  that  noted  by  Humboldt  upon  the  table-lands  of  America. 
Everywhere  analogous  differences  of  level  are  remarked ;  in  Western 
Siberia,  1°  in  450  feet  is  the  result  arrived  at;  and  this  number  is  con- 
verted into  440,  if  the  comparison  includes  the  elevated  regions  of 
Northern  India.  In  the  United  States  the  decrease  is  1°  to  400  feet. 
The  configuration  of  the  country  seems  to  be  the  most  important  ele- 
ment in  the  calculation.  If  there  is  a  gentle  rise  in  the  ground,  or  if 
the  country  is  made  up  of  successive  gradients,  the  decrease  in  the  tem- 
perature is  much  more  gradual  than  upon  the  sides  of  steep  mountains. 
In  the  first  case,  1°  may  be  taken  to  represent  a  difference  in  level  of 
420  feet;  in  the  second,  of  350  only. 


HEAT  IN  THE  ATMOSPHERE.  201 

Schouw  remarked  in  Italy,  upon  the  southern  slopes  of  the  Alps,  a 
decrease  of  1°  to  300  feet ;  less  on  Mount  Ventoux,  a  steep  and  isolated 
mountain  in  Provence  (lat.  44°  10'  K,  long.  2°  56',  height  6270  feet 
above  the  level  of  the  Mediterranean).  Martins  found,  after  nineteen 
observations,  taken  under  dissimilar  conditions,  a  decrease  of  1°  to  340 
feet  in  winter,  and  230  feet  in  summer,  or  an  average  of  260  feet.  The 
observations  of  Ramond,  made  between  43°  and  44°  of  latitude,  give  an 
average  of  1°  to  265  feet. 


202  THE  ATMOSPHERE. 


CHAPTER  III. 

THE  TEMPERATURE  OF  THE  AIR:  ITS  MEAN  CONDITION  —  DAILY  AND 
MONTHLY  VARIATIONS  OF  THE  TEMPERATURE  —  TEMPERATURE  OF 
EACH  SUMMER,  WINTER,  AND  YEAR  AT  PARIS  AND  AT  GREENWICH 
SINCE  THE  LAST  CENTURY  —  DAILY  AND  MONTHLY  VARIATIONS  OF 
THE  BAROMETER. 

WE  have  seen  that  the  earth,  by  its  annual  revolution  round  the 
sun,  and  by  its  daily  rotation  upon  its  axis,  produces  a  variation  in  the 
obliquity  of  the  solar  rays  which  find  their  way  to  it.  By  its  annual 
revolution,  they  become  more  and  more  vertical  during  six  months  of 
the  year — from  December  21  to  June  21 — and  less  and  less  so  during  the 
other  six  months.  By  its  rotation  the  horizon  each  morning  is  brought 
into  the  presence  of  the  sun,  causing  the  heat-giving  luminary  to  reign 
in  the  heights  of  the  heavens  during  the  day,  and  in  appearance  to  sink 
again  to  the  horizon  on  each  evening.  Thus,  it  is  evident  that,  by  these 
two  movements  of  the  earth,  there  are  two  general  principles  in  regard 
to  solar  heat  upon  our  planet;  the  one  annual,  the  other  diurnal. 

Let  us  consider  the  latter  first.  To  determine  it  exactly,  the  ther- 
mometer must  be  consulted  hourly,  night  and  day,  for  several  years  to- 
gether, in  order  to  distinguish  and  eliminate  the  effects  due  to  the  rota- 
tion of  the  earth  from  those  due  to  the  numerous  other  causes  which  in- 
fluence change  of  temperature.  Few  meteorologists  have  been  willing 
to  undertake  so  arduous  a  task.  Ciminello  of  Padua  made  such  obser- 
vations for  nearly  sixteen  consecutive  months.  I  say  very  nearly,  be- 
cause the  observations  at  midnight,  and  at  the  hours  of  one,  two,  and 
three  in  the  morning,  were  replaced  by  two,  taken  during  the  same  in- 
terval at  different  hours.  He  was  the  first  to  make  hourly  series  of 
thermometrical  observations.  Since  that  time  others  have  been  made 
by  Gatterer,  a  contemporary  of  his ;  by  the  artillery  officers  at  Leith ; 
by  Neuber  at  Apenrade,  in  Denmark ;  by  Lohrmann  at  Dresden ;  by 
Koller  at  Kremsmunster ;  by  Kaemtz  at  Halle;  and  at  the  Observa- 
tories of  Milan,  St.  Petersburg,  Munich,  and  Greenwich.  Such  observa- 
tions are  now  continuously  recorded  at  the  Roman  Observatory,  and 
some  others  by  means  of  a  self- registering  apparatus. 


THE  TEMPERATURE  OF  THE  AIR.  203 

The  result  of  these  observations,  and  of  many  others  which  have  been 
made  every  two  or  every  three  hours,  shows  that  the  hottest  moment  of 
the  day  is  two  in  the  afternoon,  and  the  coldest  about  half  an  hour  be- 
fore sunrise.  These  two  limits  vary  but  little  from  one  month  to  an- 
other. The  difference  of  temperature  between  the  hottest  moment  and 
the  average  coldest  period  of  the  twenty-four  hours  is  about  14°  at  Paris. 
This  amount,  however,  varies  with  the  time  of  the  year. 

The  average  yearly  maximum  temperature  at  the  Paris  Observatory 
is  58°  at  2  P.M. ;  the  average  minimum  is  44°48  at  four  in  the  morn- 
ing; and  the  average  mean  temperature  of  the  year,  as  taken  at  8'20 
A.M.  and  8-20  P.M.,  is  51° -3. 

The  interval  of  time,  between  the  minimum  in  the  morning  and  the 
maximum  in  the  afternoon,  is  only  ten  hours ;  and  the  interval  is  four- 
teen hours,  viz.,  from  2  P.M.  to  4  A.M.  between  the  time  of  maximum 
and  the  next  minimum.  The  minimum  of  the  diurnal  variation,  as 
a  rule,  takes  place  just  before  sunrise;  in  the  early  part  of  the  year  it 
is  just  before  6  A.M.,  and  occurs  earlier  as  the  days  lengthen.  After 
the  month  of  February  it  occurs  at  about  '5  A.M.,  then  at  4  A.M.,  af- 
terward oscillating  between  three  and  four  in  the  morning  during  the 
longest  days.  In  the  beginning  of  August  the  minimum  is  again  at 
4  A.M.,  returning  to  about  6  A.M.,  when  the  days  are  at  their  shortest. 
It  is  even  somewhat  later  than  this  for  a  short  period,  but  soon  after- 
ward resumes  the  annual  progress  given  above. 

The  mean  temperature  of  a  day,  in  the  mathematical  acceptation  of 
the  term,  represents  the  average  of  the  temperatures  corresponding  to 
every  instant  of  the  day.  If  the  duration  of  these  instants  be  a  minute, 
it  would  be  necessary  to  divide  the  sum  of  the  1440  thermometrical  ob- 
servations taken  between  two  consecutive  midnights  by  1440  (the  num- 
ber of  minutes  in  twenty-four  hours),  and  the  quotient  would  give  the 
required  mean  temperature.  Again,  by  dividing  by  365,  the  sum  of 
the  365  mean  temperatures  of  every  day  in  the  year,  we  should  obtain 
the  mean  annual  temperature. 

It  would  seem,  from  the  preceding  definition,  that  to  obtain  the  mean 
temperatures  accurately,  observations  at  short  intervals  would  be  indis- 
pensable ;  but  the  change  of  temperature  under  ordinary  conditions  is, 
fortunately,  of  such  a  nature  that  the  half  sum  of  the  maximum  and 
minimum  temperatures  (at  2  P.M.  and  sunrise)  is  found  to  differ  but  lit- 
tle from  the  mean  of  observation  taken  at  every  hour.  So  early  as 
1818,  Arago  pointed  out  that  the  average  temperature  at  8'20  A.M.  was 


2Q4:  THE  ATMOSPHERE. 

nearly  the  same  as  the  average  temperature  of  the  year.  Numerous 
thermometrical  observations  taken  under  his  direction  were  based  upon 
the  fact  of  the  mean  temperature  of  the  day,  occurring  twice  in  the 
course  of  the  day.  But  it  has  since  been  found  that  this  method  is  de- 
fective; for  from  8  to  9  A.M.,  and  also  from  8  to  9  P.M.,  the  temperature 
often  varies  very  rapidly.  The  averages  were  afterward  formed  by 
taking  the  temperatures  at  4  A.M.  and  10  A.M.,  and  again  at  4  P.M.  and 
10  P.M.,  adding  and  dividing  by  four.  The  arithmetical  mean  of  the 
observations  taken  at  6  A.M.,  2  P.M.,  and  10  P.M.,  also  gives  about  the 
same  value,  the  difference  being  about  two-tenths  of  a  degree.  Since 
meteorology  has  been  more  methodically  followed,  still  greater  accura- 
cy has  been  acquired,  and  it  has  been  found  that  the  twenty-four  hour- 
ly observations  may  be  replaced  by  eight  tri-hourly  observations,  taken 
at  1  A.M.,  4  A.M.,  7  A.M.,  and  10  A.M.  ;  and  at  1  P.M.,  4  P.M.,  7  P.M.,  and 
10  P.M. 

Let  us  now  consider  the  annual  movement  of  the  temperature. 

The  various  causes  which  influence  the  action  of  the  sun's  heat  vary 
but  little  throughout  the  year  in  the  regions  near  the  equator,  whether 
situated  in  the  northern  or  the  southern  hemisphere,  the  tropical  re- 
gions, as  they  are  called,  and  which  form  the  torrid  zone.  The  day  has 
about  the  same  length  all  the  year  round ;  the  meridian  height  of  the 
sun  undergoes  but  little  variation  there;  and  the  four  seasons  differ 
very  little,  in  regard  to  temperature,  the  one  from  the  other.  For  an 
entirely  opposite  cause,  the  seasons  are  very  dissimilar  both  to  the 
north  and  to  the  south  of  the  equator  in  the  regions  where  the  length 
of  the  day  varies  very  much  in  course  of  the  year,  or,  to  express  the 
same  thing  in  other  words,  where  the  meridian  height  of  the  sun  at  one 
solstice  is  very  different  from  that  of  the  other. 

We  have  considered  the  general  condition  of  the  seasons  in  our  lati- 
tudes. Let  us  now  consult  .the  figures  themselves.  The  table  append- 
ed gives  the  mean  temperature  at  the  Paris  Observatory. 

It  shows  that,  whether  the  average  maximum  or  the  average  mini- 
mum of  each  month  be  taken  into  account,  or,  indeed,  if  we  merely 
take  the  mean  temperatures  alone,  the  heat  follows  an  ascending  scale 
from  January  to  July,  and  a  descending  scale  from  July  to  December. 
The  hottest  month  is  that  of  July,  which  follows  upon  the  summer  sol- 
stice, and  the  coldest  that  of  January,  which  comes  after  the  winter  sol- 
stice. The  average  of  the  minima  is  only  once  (in  January)  below  32°  ; 
the  coldest  months  are  December,  January,  and  February,  constituting 


THE  TEMPERATURE  OF  THE  AIR. 


205 


the  real  climatological  winter ;  spring  is  made  up  of  the  months  of 
March,  April,  and  May ;  summer  of  the  three  hottest  months,  June, 
July,  and  August ;  and  the  other  three  months,  September,  October, 
and  November,  form  the  true  autumn. 

TABLE  OF  THE  MEAN  TEMPERATUEES  AT  PARIS  (ARAGO,  1806-1851). 


Months. 

Maxima. 

Minima. 

Means. 

Degrees. 
41  '0 

Degrees. 
30  -5 

Degrees. 
35  '8 

February 

45-1 

33  -3 

39  '2 

March  

50  '0 

37'7 

43  -9 

April  

55  '6 

43'7 

49  -6 

May 

65-1 

51  '3 

58  -i 

June  .. 

70  -0 

56  '5 

July.  ., 

72  -9 

59*7 

66  -2 

August  

72  -3 

58  '3 

65  '3 

September  

65  '9 

53  '8 

59  '9 

October 

58  '3 

45  '1 

51'8 

49  '5 

39-0 

44  -2 

December  . 

44  '3 

32  -5 

38  '5 

Annual  Temperatures  

57-5 

45-1 

51-3 

The  above  averages  are  those  which  Arago  arrived  at  after  forty-six 
years  of  observations  (1806-1851).  Since  then,  further  observations 
have  given  a  result  still  more  in  conformity  with  the  secular  mean  tem- 
perature of  Paris,  representing,  as  it  does,  a  longer  series  of  years. 

The  heat  received  from  the  sun  by  the  -earth  varies  inversely  as  the 
square  of  its  distance  from  the  sun ;  and  as  the  earth  does  not  move  in 
a  circular  orbit,  there  is,  in  addition  to  the  monthly  variation  caused  by 
the  inclination  of  the  solar  rays,  a  variation  due  to  the  distance  of  the 
sun.  In  fact,  we  are  farther  from  the  sun  during  the  summer  than  we 
are  in  the  winter;  and  the  difference  is  considerable.  The  following 
are  the  deviations,  taking  as  unit  the  mean  solar  distance,  and  regard- 
ing the  heat  as  reciprocal  to  the  square  of  the  distance : 


Distance. 

Solar  Heat. 

1,000,000 

1-0000 

In  Perihelion  (least  distance)        

983,208 

1-0345 

In  Aphelion  (greatest  distance)  

1,016,792 

0-9673 

Thus,  before  even  reaching  our  atmosphere,  the  solar  heat  rays  are 
subject  to  a  variation  of  nearly  one-fifteenth ;  that  is  to  say,  that  the 
solar  heat  during  winter  is,  in  respect  to  our  globe,  about  one-fifteenth 
greater  than  during  summer. 

This  difference  is  sufficiently  great  to  be  taken  into  account. 


9Q6  THE  ATMOSPHERE. 

The  diurnal  and  monthly  variations  of  temperature  increase  as  the 
distance  from  the  equator  increases.  From  the  equator  to  10°  north 
latitude,  the  mean  temperatures  of  the  various  months  scarcely  differ 
more  than  4°  to  6°.  At  20°  north  latitude  they  vary  from  10°  to  12°. 
At  30°  distance  the  regular  mean  monthly  variation  is  found  to  reach 
22°.  In  Italy  the  regular  curve  at  Palermo,  in  Sicily,  extends  from 
51°  to  74° ;  and  this  range  is  moreover  diminished  by  the  contiguity  of 
the  sea.  At  Paris  the  mean  curve  varies  from  35|°  (January)  to  66° 
(July),  and  the  changes  become  much  greater  between  the  frosts  of  win- 
ter and  the  heat  of  summer.  At  Moscow  the  mean  monthly  range 
extends  from  12°  (January)  to  75°  (July);  showing  a  difference  of 
63°  of  mean  temperature.  Lastly,  we  may  add  to  this  scale  of  varia- 
tions that  of  Boothia  Felix,  a  northern  country  of  America,  situated  be- 
yond 72°  of  north  latitude.  There  the  range  varies  from  —40°  (72° 
below  the  freezing-point  of  water)  in  February  to  41°  in  July;  exhibit- 
ing a  difference  of  81°  between  the  mean  monthly  temperatures  of  the 
year. 

The  diurnal  variation  also  gives  rise  to  remarkable  curves  in  its  suc- 
cessive temperatures.  The  range  of  thermometrical  oscillation  is  great- 
er in  warm  climates  and  inland  countries  than  it  is  in  colder  lands  and 
in  the  neighborhood  of  the  sea.  Apart  from  the  equalizing  influence 
of  the  sea,  which  remains  about  the  same  all  the  year  round,  the  dis- 
tance from  the  equator  acts  in  an  opposite  way  upon  the  annual  and 
the  diurnal  oscillations  of  the  thermometer.  While  the  first  increases 
on  account  of  the  length  of  the  nights  in  winter  and  of  the  days  in  sum- 
mer, the  second  decreases  because  in  the  southern  countries  the  heat  of 
the  sun's  rays  is  greater  and  the  sky  clearer  during  the  night.  Thus, 
for  instance,  at  Padua  the  diurnal  variation  in  July  is  about  16° ;  at 
Paris  it  is  on  an  average  about  13° ;  at  Leith  it  is  about  9°. 

These  are  the  mean  values.  But  if  the  changes  of  temperature  in  a 
given  district  be  constantly  recorded,  it  will  be  found  that,  apart  from 
these  regular  mean  variations  caused  by  the  sun,  there  are  other  varia- 
tions of  a  much  larger  amount  which  exercise  the  greatest  influence 
upon  the  public  health ;  these  are  the  diurnal  variations  that  occur  in 
the  space  of  twenty -four  hours.  These  differences  are  very  interesting, 
especially  if  we  notice  the  reading  of  a  thermometer  with  its  bulb 
placed  in  the  full  rays  of  the  sun  by  day,  and  of  another  with  its  bulb 
exposed  fully  to  the  clear  sky  at  night.  There  are  also  often  very 
great  differences  between  the  maximum  and  the  minimum  temperatures 


THE  TEMPERATURE  OF  THE  AIR. 


207 


of  the  air  of  the  same  day,  especially  in  the  months  of  May  and  June — 
differences  which  reach,  in  Paris,  to  as  much  as  45°  to  55°. 

The  following  are  some  of  the  maxima  observed  at  Montsouris,  be- 
tween 1  and  4  P.M.,  with  a  thermometer  with  green  bulb,  exposed  to 
the  sun  at  a  height  of  about  four  inches  above  grass,  as  also  some  of 
the  minima  taken  from  the  same  thermometer  between  one  and  four  the 
following  morning.  I  select  those  that  exhibit  the  greatest  differences : 


Maximum. 

Minimum. 

Difference. 

M 

ay  11  18 

70     

Degrees. 
87'3 

Degrees. 
39-4 

Degrees. 
47-9 

16, 

86-4 

42-8 

43  '6 

17, 

90-9 

44-4 

46-5 

18 

102  '9 

53'8 

49  '1 

19 

106'7 

57-9 

48'8 

20 

107'5 

55  -2 

52'3 

21, 

111-2 

60  '8 

50'4 

25, 

86-0 

41-0 

45-0 

27, 

87-4 

43-0 

44-4 

30 

94'6 

50  -4 

44  -2 

Ti 

ne    8 

86-9 

42-8 

44-1 

12. 

89-6 

46-4 

43-2 

13, 

92-5 

47'3 

45-2 

14 

107-4 

53  -6 

53  -g 

16 

106  5 

61  '0 

45-5 

23 

105'4 

53-1 

52'3 

29, 

95  2 

48-2 

47-0 

30, 

95-0 

44-8 

50-2 

J 

iiy  2, 

86-0 

42-8 

43-2 

This  shows  how  great  at  times  are  the  diurnal  variations  in  these  lat- 
itudes. The  mean  temperature  of  a  place  is  that  found  by  adding  up 
the  annual  mean  temperatures  and  dividing  their  sum  by  the  number 
of  years  during  which  the  observations  have  been  taken.  This  mode 
of  operation  is  only  applicable  to  a  limited  number  of  stations.  It  was 
necessary,  therefore,  to  seek  a  method  of  obtaining,  by  means  of  experi- 
ments which  could  be  readily  made,  approximate  mean  temperatures, 
with  a  fair  approach  to  accuracy.  We  know  that  the  surface  of  the  soil 
undergoes  daily  variations  of  temperature,  that  lower  down  there  is  a 
stratum  which  experiences  only  small  annual  variations,  and  that  at  a 
greater  depth  still,  at  about  seventy  to  eighty  feet,  there  is  a  stratum 
with  constant  temperature  which  is  found  to  be  very  nearly  the  same 
as  the  average  of  a  long  series  of  the  daily  temperatures  of  the  atmos- 
phere made  at  the  same  place.  By  finding  the  temperature  of  this  stra- 
tum at  a  sufficient  depth,  or,  which  comes  to  the  same  thing,  by  ascer- 
taining the  constant  temperature  of  springs  or  wells  in  a  certain  district, 
or  even  of  tunnels,  we  may  thus  succeed  in  obtaining  for  the  tempera- 


208 


THE  ATMOSPHERE. 


ture  of  each  place  a  number  differing  but  slightly  from  that  which 
would  be  found  by  taking  a  long  series  of  annual  temperatures  at  that 
place.  In  the  equinoctial  regions,  a  thermometer  simply  sunk  in  the 
earth  to  the  depth  of  thirteen  inches  in  sheltered. spots  will  continue  to. 
mark  the  same  degree  of  temperature  with  a  difference  of  0°  2'  or  0°  4' 
of  a  degree  at  most.  For  this  purpose  a  hole  is  dug  under  the  tents  of 
the  Indians  or  inside  a  shed,  in  a  place  where  the  ground  is  protected 
from  the  heat  caused  by  the  direct  absorption  of  the  solar  rays,  from 
nocturnal  radiation,  and  from  the  infiltration  of  rain. 

By  taking  the  temperature  of  springs  as  that  of  the  highest  stratum 
of  constant  temperature,  there  will  be  found  a  great  similarity  in  re- 
spect to  the  zone  comprised  between  30°  and  55°  latitude,  provided 
that  the  places  are  not  more  than  3000  feet  above  the  level  of  the  sea. 

In  respect  to  latitudes  above  55°,  the  difference  between  the  tempera- 
ture of  the  air  and  of  springs  increases  to  a  marked  extent. 

Toward  the  peak  of  the  Swiss  Alps,  above  an  elevation  of  from  4600 
to  4900  feet,  as  in  the  high  latitudes,  the  springs  are  nearly  6°  Fahr. 
warmer  than  the  air. 

In  Southern  countries  the  temperatures  of  springs  and  of  the  ground 
are  less  than  the  mean  temperatures  of  the  air,  as  may  be  gathered  from 
the  accounts  of  Humboldt  and  Leopold  von  Buch. 

In  our  latitudes  this  temperature  is  equal  to  that  of  the  soil  near  the 
surface,  and  is  a  little  higher  than  the  average  of  the  particular  place. 

It  is  worth  our  while  to  complete  this  general  study  of  the  meteorol- 
ogy of  our  climate  by  enumerating  the  mean  temperatures  at  Paris  and  at 
Greenwich  since  the  beginning  of  the  present  century.  They  are  furnished 
from  the  archives  of  the  Observatories  at  Paris  and  at  Greenwich. 

MEAN  TEMPERATTJKES  AS  DETERMINED  AT  THE  PARIS  OBSERVATORY  AND  AT  THE 
ROYAL  OBSERVATORY,  GREENWICH. 


Winter. 

Summer. 

(Dec.,  Jan.,  Feb.) 

(June,  July,  August.) 

Years. 

Paris. 

London. 

Paris. 

London. 

Paris. 

London. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

1800 



34-6 



60-7 

50-4 

48-3 

1801 



38'7 



60-5 

51'3 

49-0 

1802 



36-0 



59-6 

50-0 

48-0 

1803 

— 

35-8 



60-5 

51-1 

48-2 

1804 

41'0 

41-1 

65-5 

60-5 

52-0 

49-5 

1805 

36-0 

36'3 

63-1 

58-4 

49-5 

47-7 

1806 

40-8 

40-5 

65'5 

60-8 

52-4 

50-5 

1807 

42-8 

41-2 

67-3 

61-6 

51-4 

48-3 

1808 

35-8 

36'6 

66-6 

62-1 

50-7 

48-1 

1809 

40-8 

38-5 

62-4 

58-7 

51-1 

48-0 

1810 

35'6 

38-0 

63-5 

60-0 

611 

48-7 

THE  TEMPERATURE  OF  THE  AIR. 


209 


Winter. 
(Dec.,  Jan.,  Feb.) 

Snmmer. 
(June,  July,  August.) 

Year?. 

Paris. 

London. 

Paris. 

London. 

Paris. 

London. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

1811 

39-2 

37-2 

64-6 

59-0 

53-6 

49-6 

1812 

39-4 

38-6 

63-0 

56-1 

48-2 

46-5 

1813 

35-1 

37-0 

61-7 

57'5 

50-4 

47-2 

1814 

33-8 

32-5 

63-3 

57-7 

50-2 

45-8 

1815 

39-7 

38-1 

62  '8 

59'4 

50-9 

49-0 

1816 

36-0 

36-8 

59-5 

55-2 

48-9 

46-4 

1817 

41-4 

39'9 

62-8 

57-4 

50-9 

47-7 

1818 

38-3 

37-4 

66-6 

64-2 

52-3 

50-8 

1819 

39-6 

39-6 

64-8 

60-6 

52-0 

49-3 

1820 

35-4 

35-2 

63-5 

58-0 

50-6 

47-4 

1821 

36-5 

37-8 

63-0 

57-8 

52-0 

49-3 

1822 

42-8 

42-5 

67'5 

62-1 

53-8 

51-0 

1823 

34-7 

35-4 

62-8 

58-0 

50-7 

47-3 

1824 

39-9 

37-8 

64-0 

59-2 

52-2 

48-3 

1825 

41-0 

39-4 

66-0 

62-0 

53-1 

49-6 

1826 

38-7 

38-3 

68-4 

63-9 

52-5 

49-9 

1827 

34-7 

35-6 

64-8 

60-0 

51-4 

48-5 

1828 

42-8 

41-4 

64-6 

60-3 

52-7 

50-1 

1829 

35-2 

38-2 

63-7 

58-9 

48-4 

46-6 

1830 

28-8 

31-9 

63-1 

58-8 

50-2 

47-8 

1831 

38-7 

36-8     ' 

64-6 

62-3 

53-1 

50-4 

1832 

38-3 

38-7 

66-6 

60-5 

51-4 

49-1 

1833 

38-7 

39-8 

63-9 

59-5 

51-6 

49-0 

1834 

43-3 

43-1 

66'6 

62-5 

54-1 

51-0 

1835 

39-9 

40-1 

66-8 

62-6 

51-3 

49-2 

1836 

35-4 

36'3 

65'5 

60-3 

51-3 

48-1 

1837 

39-0 

39-0 

66-0 

59-8 

50'0 

47-3 

1838 

33-3 

34-3 

63-3 

59-1 

48-6 

46-4 

1839 

37-8 

"  38-3 

65'1 

59-3 

51-6 

47-7 

1840 

39-6 

38-9 

65-1 

59-9 

50-5 

47-8 

1841 

33-3 

34-1 

62-1 

58-3 

52-2 

48-7 

1842 

37-2 

38-1 

69-4 

62-8 

51-8 

49-6 

1843 

39-4 

40-3 

64-2 

59-8 

52'3 

49-4 

1844 

37-8 

39-4 

62'2 

59-9 

50-4 

48-7 

1845 

32-7 

34-7 

62'6 

59-3 

49-5 

47-6 

1846 

42-4 

43-1 

69'1 

64-3 

53-1 

51-3 

1847 

35-1 

34-5 

65-1 

61-8 

51-4 

49-6 

1848 

37-9 

40-3 

64-8 

59-5 

52-5 

50-2 

1849 

42-8 

42-4 

64-8 

61-0 

52-3 

49-9 

1850 

38-8 

39-2 

65-3 

61-1 

611 

49'3 

1851 

39-6 

41-2 

64-9 

60-4 

50-9 

49-2 

1852 

39-0 

41-1 

66-7 

61-6 

53-1 

50-6 

1853 

41-4 

41-1 

63-9 

59-5 

50-2 

47-7 

1854 

37-5 

37-5 

63-0 

58-9 

51-6 

49-0 

1855 

35-8 

35-2 

64-0 

60-4 

49-1 

47-8 

1856 

39-4 

39-0 

66'0 

611 

51-4 

49-0 

1857 

37'8 

38-7 

66-7 

64-0 

52-3 

51-1 

1858 

36-3 

39-1 

66-6 

62-5 

50-7 

49-2 

1859 

40-1 

41-5 

67-1 

64-3 

52-5 

50-8 

1860 

36-5 

37-4 

61-2 

56-7 

48-6 

47-0 

1861 

36-0 

37-4 

65-5 

611 

51-3 

49-4 

1862 

39-0 

40-4 

62-4 

58-3 

51-3 

49-6 

1863 

41-2 

42-5 

65'5 

60-3 

52-5 

50-3 

1864 

37-6 

38-6 

63-1 

59-6 

49-8 

48-5 

1865 

36-1 

37-8 

65-1 

61-3 

.    52-5 

,     50-3 

1866 

40-5 

41-9 

64'2 

60-4 

52-0 

49-8 

1867 

41-2 

40*6 

63-7 

59-8 

50-9 

48-6 

1868 

36-9 

39-2 

67-3 

64-4 

53-2 

51-6 

1869 

43-7 

44-1 

63  '3 

60-2 

51-3 

49-5 

1870 

36-5 

37-5 

66-0 

62T> 

50-4 

48-7 

14 


21Q  THE  ATMOSPHERE. 

The  preceding  table  shows  that  the  coldest  winter  of  the  present 
century  in  Paris  was  that  of  1830 ;  the  mildest,  that  of  1869 ;  the  cold- 
est summer,  that  of  1816 ;  the  hottest,  that  of  1842  ;  the  coldest  year 
was  1829 ;  and  the  hottest,  1834.* 

This  list  gives  the  mean  annual  temperature  of  winter  and  summer, 
as  ascertained  at  the  Paris  Observatory.  We  shall  see  farther  on  that 
there  have  been  more  severe  frosts  and  greater  heat  in  France  than 
those  given,  but  they  have  been  observed  at  different  places. 

I  have  already  stated  that,  taking  the  mean  temperatures  of  each  day 
of  the  year  at  Paris,  it  would  be  seen  that  there  is  an  increase  in  heat 
from  the  first  week  in  January  to  the  middle  of  July,  with  a  continuous 
decrease  from  the  latter  date  until  the  close  of  the  year.  The  general 
phenomenon,  however,  exhibits  certain  discontinuities  which  can  not 
be  treated  so  simply. 

It  is  true  that,  generally  speaking,  it  is  the  movement  of  the  earth 
which  gives  rise  to  the  grand  phases  of  the  temperature,  and  which 
produces  in  our  climates,  for  instance,  a  minimum  in  January  and  a 
maximum  in  July.  But  the  curve  which  unites  these  two  extreme 
points  is  not  regular.  There  are  unmistakable  departures  from  conti- 
nuity which  seem  subject  to  periodical  returns. 

In  its  more  general  aspect,  the  question  mayTDe  put  in  the  following 
manner : 

What  is,  for  a  given  locality,  the  mean  departure  which  the  tempera- 
ture of  each  day  in  the  year  exhibits  in  relation  to  the  supposed  regular 
march  of  these  temperatures  between  the  annual  extremes? 

[*  M.  Flammarion  has  given  this  table  for  Paris  only ;  I  have  added  the  corresponding 
values  for  Greenwich,  as  taken  from  my  paper  in  the  Philosophical  Transactions  for  the  year 
1848,  supplemented  by  subsequent  results.  I  may  remark  that  I  have  altered  some  values 
in  M.  Flammarion's  table  as  seemed  to  be  necessary  by  comparison  with  the  tables  in  the 
"Annuaire"  for  1872. 

This  table  shows  that  the  coldest  and  warmest  winters,  the  coldest  summer,  and  the  coldest 
year,  were  the  same  at  Paris  and  at  Greenwich,  and  that  the  warmest  summer  and  the  warm- 
est year  at  Greenwich  was  1868. 

It  also  shows  that  the  most  severe  winter  of  all  was  at  Paris,  and  that  the  winter  tempera- 
ture of  Paris  is  frequently  lower  than  at  Greenwich,  although  generally  it  is  higher. 

The  mean  temperature  of  the  winter  at  Paris  from  all  the  years  is  38°  "4,  while  that  at 
Greenwich  is  37°'l. 

The  mean  temperature  of  the  summer  months  has  in  every  case  been  warmer  at  Paris  than 
at  Greenwich.  * 

The  mean  of  summer  at  Paris  is  64° '7 ;  at  Greenwich,  60° '4. 

The  mean  temperature  of  every  year  is  higher  at  Paris'  than  at  Greenwich :  the  mean  at 
Paris  is  51°'3 ;  that  at  Greenwich,  48°'9.—  ED.] 


THE  TEMPERATURE  OF  THE  AIR.  211 

Is  this  departure  about  the  same  for  each  year,  or  for  a  small  group 
of  years — or  does  it,  on  the  contrary,  vary  from  one  year  to  another,  or 
from  one  group  of  years  to  another,  so  as  to  present  a  certain  periodical 
recurrence  ? 

The  questions,  which  are  secondary  to  the  first  general  question,  are 
very  numerous,  inasmuch  as  the  quantities  of  light  which  enter  into  the 
atmosphere,  the  electric  state  of  the  air,  and  its  so-called  ozonometrical 
properties,  its  hygrometrical  condition,  as  also  the  variations  in  the  at- 
mospheric pressure,  the  displacement  of  the  air,  or  the  winds  and  tem- 
pests— in  a  word,  all  the  atmospheric  phenomena  are  intimately  bound 
up  with  the  distribution  of  heat  over  the  surface  of  the  globe. 

Lastly,  a  very  natural  and  important  addition  consists  in  the  influ- 
ence of  these  thermometrical  perturbations  upon  the  health  of  men,  ani- 
mals, and  of  plants. 

Four  epochs  in  the  year  are  remarkable  for  a  fall  in  the  temperature, 
and  atmospheric  perturbations  caused  thereby,  viz.,  about  the  12th  of 
February,  the  12th  of  May,  the  12th  of  August,  and  the  12th  of  No- 
vember. 

The  periodical  cold  of  the  month  of  May  is  a  popular  tradition  ;  hor- 
ticulturists term  St.'Mamert,  St.  Pancras,  and  St.  Servais,  whose  anniver- 
saries are  on  the  llth,  12th,  and  13th  of  May,  the  three  ice-saints. 

In  February  there  are  the  same  indications,  but  they  are  even  more 
marked.  The  fall  after  the  7th  of  February  is  very  sudden,  and  con- 
tinues to  the  12th,  which  gives  but  a  single  minimum  even  in  the  mid- 
dle of  the  ice-saints  of  February.  As  February  with  us  represents 
Northern  climates,  every  thing  will  be  extreme,  the  rise  as  well  as  the 
fall ;  in  August,  on  the  other  hand,  which  gives  us  an  idea  of  the  trop- 
ical climates,  the  changes  are  less  sudden,  and  the  slight  movement  cor- 
responding to  that  of  the  10th  to  the  14th  in  May,  or,  in  another  form, 
of  the  August  ice-saints,  continues  until  the  16th. 

In  November,  as  in  August,  the  decline  of  the  temperature  is  seen  to 
be  struggling  against  influences  which  tend  to  an  abnormal  return  of 
heat ;  the  points  of  inflection  correspond  precisely  to  those  of  the  three 
other  months,  and  one  of  the  last  of  them  produces,  on  the  14th,  the 
Martinmas  summer. 

The  careful  examination  of  a  large  number  of  years  shows  that,  at 
London  and  Berlin,  as  at  Paris,  there  is  a  certain  agreement  between  the 
four  days  of  the  same  date,  as  exhibited  in  their  mean  temperatures.  M. 
Ch.  Sainte-Claire  Deville  ascertained  that  these  curious  periods  are  to  be 


212  THE  ATMOSPHERE. 

found  in  the  most  ancient  of  known  meteorological  documents;  for  in- 
stance, in  the  observations  of  the  pupils  of  Galileo,  and  of  the  Academy 
of  Cimento.  These  observations  extend  over  fifteen  years  (1655-1670). 
The  minimum  of  the  ice-saints  is  found  to  occur  on  the  12th,  with  a 
remarkable  regularity. 

It  is  certain  for  the  last  two  centuries,  in  this  part  of  Europe,  that  the 
periodic  anomalies  of  the  temperature,  some  of  which  were  proverbial 
among  our  ancestors,  have  manifested  themselves  in  the  same  manner 
stated  above. 

Certain  astronomers,  Erman  and  Petit  among  the  number,  have  at- 
tributed these  frigorific  phenomena  to  masses  of  asteroids  which,  in  their 
orbit,  sometimes  come  between  the  sun  and  the  earth. 

The  action  of  the  sun  produces,  therefore,  in  the  temperature  of  the 
air,  variations  according  to  the  hours  of  the  day  and  the  month  of  the 
year.  This  same  solar  action  produces  a  diurnal  and  a  monthly  varia- 
tion in  the  readings  of  the  barometer,  which,  perhaps,  had  better  be  con- 
sidered here,  as  it  is  a  consequence  of  the  temperature. 

The  atmospheric  pressure  increases  and  decreases  twice  each  day  with 
regularity,  in  a  manner  dependent  on  the  sun's  position.  The  reading 
of  the  barometer,  which  shows  the  weight  of  the  atmosphere,  gradu- 
ally increases  from  4  to  10  A.M.  This  atmospheric  tide  is  not  due,  like 
that  of  the  sea,  to  the  attraction  of  the  moon  and  the  sun,  since  it  takes 
.place  every  day  at  the  same  hour,  and  does  not  follow  the  course  of  the 
moon.  It  is  due  to  the  expansion  produced  by  the  solar  heat,  and  to 
the  increase  in  the  vapor  of  water,  also  produced  by  this  same  heat. 

This  barometrical  variation  is  not  great,  for  it  never  attains  so  much 
as  one-tenth  of  an  inch.  It  was  about  the  year  1722  that  the  diurnal 
variations  of  the  barometer  were  first  ascertained  by  a  Dutchman,  whose 
name  has  not  reached  us.  Since  that  epoch,  several  observers  have  en- 
deavored to  determine  their  amounts  and  their  periods  for  different 
parts  of  the  earth.  Humboldt  proved,  by  a  long  series  of  observations, 
that,  at  the  equator,  the  maximum  of  elevation  corresponds  with  9  A.M.; 
after  that  hour,  the  barometer  reading  decreases  until  4  or  3'30  P.M., 
when  it  attains  its  minimum.  It  afterward  increases  again  until  11  P.M., 
when  it  reaches  a  second  maximum,  and,  lastly,  decreases  again  until  4 
A.M.  Thus  there  are  each  day  two  minima,  at  4  A.M.  and  4  P.M.  ;  and 
two  maxima,  at  9  A.M.  and  11  P.M.  The  movements  are  so  regular  that 
a  simple  glance  at  the  barometer  suffices  to  ascertain  the  hour,  especial- 
ly during  the  day,  without  any  probability  of  being  more  than  a  quar 


THE  TEMPERATURE  OF  THE  AIR. 


213 


ter  of  an  hour  in  error.     They  are  so  permanent  that  neither  tempest, 

nor  storm,  nor  rain, 


3    4   S   6   7    8 


fects  it  ;  they  main- 
tain themselves  as 
steady  in  the  warm 
regions  of  the  coast 
of  the  New  World 
as  upon  table  -lands 
more  than  13,000 
feet  high,  where  the 
mean  temperature 
falls  to  44£°.  The 
amount  of  the  os- 
cillations diminish- 
es as  the  latitude  in- 
creases, in  the  same 
manner  as  the  mean 
temperature  of  a 
place  is,  in  general, 
higher  the  nearer  it 
is  to  the  equator. 

At  the  Antilles,  it 
is  found  that  there 
is  a  distinctly-mark- 
ed maximum  for  the 
diurnal  oscillation 
along  the  northern 
coast  of  America, 
which  is  situated  op- 
posite to  the  sea  of 
the  Antilles.  The 
stations  upon  this 
coast  -line  give,  on 
an  average,  an  am- 
plitude of  O'll  inch- 
es; whereas  at  all 
the  other  stations 

Fig.  50.—  Regular  diurnal  oscillation  of  the  Barometer:  1.  Ascension  Isl- 

and ;  2.  Port  d'Espagne  ;  3.  Acapulco  ;  4.  Cumana;  5.  Basse-Terre.        the  amount  is  Small- 


214 


THE  ATMOSPHERE. 


er,  whether  they  are  situated  to  the  north  or  south  of  the  littoral  region 
in  question. 

The  northern  coasts  of  Venezuela  and  New  Grenada  are  exactly  those 
which  the  thermal  equator  follows,  rising  in  this  district  to  the  twelfth 
degree  of  north  latitude,  whence  it  descends  again  toward  the  equator, 
on  both  sides  of  the  continent.  The  place  of  the  maxima  oscillations 
of  the  barometer  is,  therefore,  the  same  as  that  of  the  maxima  tempera- 
tures, and  the  two  phenomena  follow  a  similar  march  in  the  intertropic- 
al  American  zone.  This  is,  moreover,  quite  in  accord  with  the  causes 
which  influence  the  distribution  of  temperature  over  the  different  hours 
of  the  day. 

Various  observations  have  made  it  evident  that  the  amplitude  of  the 
total  oscillation  diminishes  with  increased  altitude.  It  may  be  stated 
as  a  general  rule  that  this  amplitude  is  dependent  on  the  mean  tempera- 
ture of  the  place,  and  that  it  decreases  with  it  not  only  according  to  the 
vertical  co-ordinate  of  the  altitude,  but  according  to  the  two  co-ordinates 
of  latitude  and  longitude. 

The  diurnal  oscillation  of  the  barometer  varies  with  the  latitude  as 
follows : 


Places. 

Latitude. 

Mean  Height. 

Diurnal 
Oscillation. 

Lima  

Degrees. 
12'3     S. 

Inches. 
29-202 

Inches. 
0'107 

10  '31  N 

26*848 

0*085 

Payta  . 

5'6     S 

29'841 

0-082 

Santa-Fe  de  Bogota 

4-36  N 

29-918 

0*080 

Ibague  

4-28 

25-934 

0*076 

Calcutta  

22-35 

29-877 

0'072 

10-28 

29*770 

0*070 

Rio  de  Janeiro 

22-54  S 

30-117 

0*067 

Mexico  

1  9  "26  N 

22*958 

0*063 

Cairo  

30'2 

29-816 

0*061 

Rome 

41  '54 

29*971 

0*040 

Bale 

47-34 

29'087 

0*033 

Brussels  .  . 

50-50 

29  "806 

0*032 

Paris...  

48'50 

29-757 

0*028 

Frankfort  

50  '8 

29*626 

0*022 

Dresden  

51'7 

29'310 

0*019 

Berlin.  .  . 

52  '33 

29  "869 

0"013 

Cracow  

50'4 

29  "228 

0'012 

Edinburgh  

55-55 

29  "406 

0*008 

Konigsberg  

54-42 

29'956 

0*007 

St.  Petersburg  

59-56 

29-895 

0.005 

The  last  column  of  this  table  shows  that  at  60°  of  latitude  the  diurnal 
barometrical  oscillation  is  very  small. 

In  our  climates  these  hourly  variations  are  so  masked  by  accidental 


THE  TEMPERATURE  OF  THE  AIR. 


215 


variations  that  to  discover  and  measure  them  was  a  work  requiring  the 
greatest  sagacity  and  precision.  It  is  only  by  the  average  of  many 
years'  observations  taken  with  care  and  at  suitable  hours  that  the  hour- 
ly periods  can  be  arrived  at. 

The  following  table  gives  the  diurnal  and  monthly  atmospheric  vari- 
ation due  to  the  expansion  of  air  by  solar  heat,  as  found  from  observa- 
tions at  the  Paris  Observatory : 


Month.  • 

Mean  Heights  of  the  Barometer  reduced  to  the 
Temperature  of  0°. 

At  9  A.M. 

At  Noon. 

At  3  P.M. 

At  9  P.M. 

Inches.  ' 
29-813 
29-798 
29-773 
29-705 
29-738 
29-787 
29-786 
29-780 
29-773 
29-756 
29-738 
29-816 

Inches. 
29-810 
29-782 
29-764 
29-690 
29-727 
29-777 
29-773 
29-766 
29-761 
29-745 
29-727 
29-796 

Inches. 
29-786 
29-768 
29-740 
29-678 
29-707 
29-758 
29-764 
29-749 
29-741 
29-725 
29-711 
29-795 

Inches. 
29-799 
29-782 
29-761 
29-694 
29-725 
29-773 
29-777 
29-768 
29-761 
29-745 
29-729 
29-813 

February  .    . 

March  

April  

May 

July. 

August  

September  

October  

December  

Means  of  the  Year  

29-772 

,29-759 

29-743 

29-760 

This  table  shows  that  the  morning  maximum  attains  on  an  average 
29772  inches,  and  the  afternoon  minimum  29*743  inches:  the  differ- 
ence is  0-029  inch.  It,  moreover,  shows  that  there  is  not  only  a  diur- 
nal variation  of  the  barometer,  but  also  a  monthly  variation. 

The  atmospheric  pressure  decreases  gradually  from  January  to  April, 
increases  a  little  up  to  July,  decreases  a  little  until  November,  and  then 
increases  in  December  and  January.  This  movement  of  the  atmospher- 
ic pressure  is  almost  the  exact  opposite  of  that  of  the  temperature ;  it  is 
much  more  marked  in  the  tropical  regions,  as  may  be  seen  by  consult- 
ing the  curves  which  M.  Deville  traced  in  the  Antilles.  The  amplitude 
of  the  monthly  oscillation  is  there  on  an  average  (29'81— 29*69=)  0-12 
inch,  between  January  and  April,  according  to  observations  taken  at 
noon.  The  nearer  one  approaches  the  tropics,  the  greater  it  is :  corre- 
spondents at  the  Calcutta  Institute  inform  me  that  0'7  inch  represent 
the  amplitude  between  January  and  July,  and  at  Benares  0'6  inch. 

The  observations  at  Brussels  show  that  the  diurnal  and  monthly  va- 
riations in  our  climates  are  distinct.  By  comparing  them  it  is  seen  that 
the  diurnal  maxima  of  temperature  are  pretty  constant  during  the  year, 
occurring  about  10  A.M.  and  10  P.M.  As  to  the  minima,  the  interval 


216 


THE  ATMOSPHERE. 


Jan.  Pel).  Mar.  Apr  May.  Jime.  Jiily.  Aug.  Sept.  Oct.  WOT.  Dec  Jan 


\/ 


between  them  is  greater  in  summer  than  in  winter;  the  two  quantities 
also  exhibit  a  greater  deviation  in  the  summer  months.     During  the 

shortest  days  (Novem- 
ber, December,  January), 
there  are  only  eight 
hours  between  the  mini- 
ma, which  occur  at  6  A.M. 
and  2  P.M.,  whereas  dur- 
ing the  other  months  the 
interval  between  them  is 
longer. 

The  time  at  which 
the  first  minimum  takes 
place  varies  more  than 
two  hours,  being  at  8'30 
A.M.  in  June,  and  6'22 
A.M.  in  December. 

There  is  an  equally 
great  change  in  the  time 
of  the  first  maximum. 
The  extreme  limits  take 
place  at  1O50  A.M.  in 
February,  and  at  8*40 
A.  M.  in  June.  Local 
causes  exercise  a  certain 
influence  upon  the  epochs 
of  these  extreme  limits. 

The  epoch  of  the  sec- 
ond minimum  varies  still 
more,  as  it  occurs  at  215 
P.M.  in  January,  and  at 
5'30  P.M.  in  June,  show- 
ing a  difference  of  time 
of  three  and  a  quarter 
hours.  The  limits  with- 
in which  the  baromet- 


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J 

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i 

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Fig.  51.—  Regular  monthly  oscillation  of  the  Barometer  :  1.  Cay- 
enne ;  2.  Guiana;  3.  Trinidad;  4.  Santa-Pe  de  Bogota-  5. 
Guadaloupe. 

imum  and  the  first  minimum 


TlCal  epoch  Varies  are,  in 

the  case  of  the  first  max- 
about two  hours.     The  interval  of  time 


THE  TEMPERATURE  OF  THE  AIR.  217 

which  elapses  between  the  first  maximum  and  the  second  minimum  de- 
serves especial  attention,  there  being  a  separation  of  four  hours  only  in 
January,  which  increases  to  eight  in  June,  the  latter  being  the  double 
of  the  former.  The  results  show  that  the  total  diurnal  variation  is 
made  up  by  the  combination  of  two  waves :  the  one  scarcely  percepti- 
ble, which,  in  the  space  of  twenty-four  hours,  has  a  maximum  and  a 
minimum  of  0'009  inch  only;  the  other  much  greater,  with  two  maxi- 
ma and  two  minima  of  O'Ol  inch. 

Such  are  the  regular  variations  of  the  barometer,  due  to  diurnal  and 
annual  action  of  solar  heat.  These  are  the  least  important  variations. 
The  atmosphere  is  unceasingly  in  movement  by  influences  which  ac- 
quire a  greater  intensity,  although  they  have  the  same  origin.  The 
irregular  variations  are  larger,  and  increase  from  the  Equator  to  the 
Poles.  While  the  extreme  differences  of  the  barometer  do  not  exceed 
upon  an  average  two-tenths  of  an  inch  in  the  equinoctial  regions  (ex- 
clusive of  the  cyclones,  which  will  be  alluded  to  hereafter),  they  reach 
to  two  and  three  inches  in  our  latitudes. 

The  greatest  barometrical  variations  occur  in  winter,  the  smallest  in 
summer. 

At  all  times  of  the  year  the  barometer  reading  is  higher  during  the 
minima  of  temperature  than  during  the  maxima. 

It  is  especially  during  autumn  and  winter  nights  that  the  differences 
of  temperature  have  the  greatest  effect  on  the  reading  of  the  barometer. 
In  spring  this  influence  is  much  less,  and  is  to  a  great  extent  disguised 
by  other  causes. 


218  THE  ATMOSPHERE. 


CHAPTER  IV. 

REMARKABLE  SUMMERS— THE   HIGHEST  KNOWN  TEMPERATURES. 

THE  first  summer  of  the  present  century,  or,  to  speak  more  exactly, 
according  to  chronology,  the  summer  of  the  last  year  of  the  eighteenth 
century,  was  remarkable  for  its  high  temperature,  and  I  might  com- 
mence the  series  with  it  but  for  the  fact  that  Europe  experienced  an 
exceptional  degree  of  heat  at  a  date  which  will  remain  famous,  that  of 
1793. 

The  summer  of  this  year  was  memorable  for  the  intense  and  unexam- 
pled heat,  which  occurred  in  July  and  August.  According  to  Cassini 
IV.,  then  director  of  the  Observatory,  the  results  for  Paris  were : 

Great  heat  (77°  to  88°) 36  days. 

Very  great  heat  (90°  to  93°) , 9     " 

Extraordinary  heat  (95°  or  higher) 6     " 

The  highest  temperatures  occurred  as. follows: 

Valence,  July  11 lOl'O  degrees. 

Paris,  July  8 lOl'l 

Paris,  August  16 99'1 

Chartres,  August  8 100-4  " 

Chartres,  August  16 100'6  " 

Verona,  July  and  August 96'1  " 

London,  July  16 89'1  " 

The  great  heat  began  in  Paris  about  the  1st  of  July,  and  increased 
very  rapidly.  The  sky  remained  continually  clear  and  cloudless;  the 
wind  was  in  the  north  and  generally  very  gentle,  and  the  barometer  re- 
mained very  high.  The  hottest  days  were  the  8th  and  16th  of  July. 
On  the  9th,  a  fearful  thunder-storm  raged  at  Senlis  and  the  immediate 
neighborhood.  Hailstones  as  large  as  eggs  destroyed  the  crops ;  a  tre- 
mendous wind  blew  down  more  than  120  houses.  This  tempest  was 
followed  by  very  heavy  rain,  and  the  water,  collecting  in  the  fields, 
swept  off  cattle,  furniture,  women,  and  children.  At  Bougueval,  in  the 
Oise,  a  woman,  after  rescuing  her  nine  children,  was  swept  off  by  the 
flood.  On  the  10th  of  July,  to  complete  the  destruction,  there  came  a 
second  hailstorm. 

The  extreme  heat  of  the  month  of  July  continued  through  part  of 


HIGH  TEMPERATURES.  219 

August.  On  the  7th  of  this  latter  month  it  was  very  great,  very  gen- 
eral, and  most  oppressive.  The  sky  was  still  clear,  and  the  wind,  from 
the  north-east,  was  so  scorching  that  it  seemed  as  if  emitted  from  the 
mouth  of  a  furnace.  It  came  by  whiffs,  and  was  as  severe  in  the  shade 
as  in  the  sun.  This  was  experienced  not  only  throughout  Paris,  but 
in  the  country  districts  as  well.  The  suffocating  heat  paralyzed  the 
breathing,  and  was  felt  far  more  severely  on  that  day,  with  the  ther- 
mometer at  86° -6  Fahr.,  than  on  the  8th  of  July,  when  it  was  101°-2. 

The  dryness  of  the  ground  became  very  great.  The  level  of  the  wa- 
ters of  the  Seine  fell  to  the  lowest  water-mark  of  1719,  at  the  end  of 
August  and  in  the  middle  of  September.  There  fell  in  Paris  but  1O9 
inches  of  rain  in  the  year.  In  the  country,  trees  and  shrubs  were  gen- 
erally burned  up;  and  fruits,  including  apples  among  them,  showed  signs 
of  having  been  burned.  There  was  a  great  scarcity  of  vegetables.  The 
land,  dried  up,  hardened  and  cracked :  it  was  impenetrable  both  to  the 
plow  and  the  spade.  Workmen  engaged  in  sinking  a  well  in  a  place 
exposed  to  the  sun,  found  the  soil  dried  up  to  a  depth  of  more  than  five 
feet.  By  the  1st  of  September  the  trees  had  lost  nearly  all  their  leaves ; 
the  dryness  and  the  heat  had  caused  the  bark  to  crack  and  the  branches 
to  look  dead.  Yery  many  of  the  trees  did,  in  fact,  die. 

In  Burgundy  the  vintage  began  on  the  23d  of  September;  the  wine 
was  abundant,  but  of  inferior  quality,  as  the  vines  had  been  affected  by 
a  cold  rain  which  fell  in  that  district.  The  summer  was  dry  and  hot  in 
the  neighborhood  of  Toulouse,  and  the  maize  crop  was  a  complete  fail- 
ure. It  will  be  remembered  that  1793  was  a  year  of  great  scarcity  in 
France. 

1800. — The  summer  was  marked  by  extreme  heat,  which  extended 
over  part  of  Europe.  From  the  6th  of  July  to  the  21st  of  August  the 
temperature  decreased  but  five  times  below  74°.2 ;  there  were,  according 
to  Bouvard,  of 

Great  heat 25  days. 

Very  great  heat / 5    " 

Extraordinary  heat 2    " 

The  highest  temperatures  were  as  follows : 

Bordeaux,  August  6 101*8  degrees. 

Nantes,  August  18 101'8        " 

Montmorency,  August  18 100-2       " 

Limoges 99'5       " 

Paris,  August  18 95'9       " 

London,  August  2 88'0 


220  THE  ATMOSPHERE. 

Conflagrations  were  very  numerous  in  the  early  part  of  April.  A 
whole  village  in  the  department  of  the  Eure,  the  forest  of  Haguenau, 
and  part  of  the  Black  Forest,  became  a  prey  to  the  flames.  Myriads  of 
grasshoppers  alighted  in  the  neighborhood  of  Strasbourg.  In  the  night 
of  July  20th  the  ancient  monastery  of  the  Augustins,  in  Paris,  was 
struck  by  lightning.  In  the  south  there  occurred  numerous  cases  of 
sudden  madness. 

1811. — The  summer  of  1811  was  in  many  respects  one  of  the  most 
memorable  known  in  Northern  Europe. 

The  following  is  the  table  of  the  maxima  temperatures : 

Augsburg,  July  30 99'5  degrees. 

Vienna,  July  6 96'3 

Avignon,  July  27 95'0 

Hamburg,  July  19 94'6  " 

Naples,  July  20 94'3  " 

Copenhagen,  July 92'8  " 

Liege 92'7 

Strasbourg 91 '4  " 

St.  Petersburg,  June  27 88'0  " 

Paris,  July  19 87'8  " 

In  Burgundy  the  vintage  began  on  the  14th  of  September.  A  hail- 
storm that  occurred  on  the  llth  of  April  spoiled  two-thirds  of  the  crop; 
but  the  summer  was  very  favorable  for  vines,  and  the  small  crop  yield- 
ed wine  of  an  excellent  quality,  which  was  long  famous  as  the  comet 
wine. 

1822. — The  summer  of  1822  was  remarkable  throughout  France  for 
high  mean  temperature. 

At  Paris  there  were  of 

Great  heat 55  days. 

Very  great  heat 3     " 

The  maxima  of  temperature  were  as  follows : 

Malines,  in  July 101 '8  degrees. 

Joyeuse,  June  23 99'1        " 

Alais,  June  14  and  23 97'7        " 

Liege 95 '0       " 

Maastricht,  June  11 93'2       " 

Paris,  June  10 92-8        " 

The  drought  was  very  great  in  France  during  the  warm  season  :  from 
the  21st  of  August  to  the  26th  of  September  the  Seine  was  nearly  con- 
tinuously below  the  mark  of  zero  at  the  Pont  de  la  Tournelle.  As  early 
as  the  month  of  March  there  was  a  scarcity  of  water ;  for  cattle  in  the 


HIGH  TEMPERATURES.  221 

south  of  France  water  had  to  be  brought  from  great  distances  upon  the 
backs  of  mules,  and  the  spring  temperature  in  that  country  was  as  high 
as  that  generally  experienced  in  August.  The  harvest  was  finished  in 
Languedoc  by  the  23d  of  June,  and  though  there  was  very  little  straw 
the  ears  were  well  filled.  In  Burgundy  the  sky  was  unusually  clear. 
The  vintage  began  on  the  2d  of  September,  but,  according  to  the  vine- 
dressers, it  might  have  been  begun  on  the  15th  of  August,  and  in  the 
neighborhood  of  Yesoul  the  grapes  were  gathered  on  the  19th  of  Au- 
gust. There  was  an  average  quantity  of  wine,  and  the  quality  was  ex- 
ceptionally good ;  the  grain  crops  were,  as  a  rule,  less  abundant  than 
usual. 

1826. — A  very  hot  and  dry  summer:  thirty-six  days  of  great  heat  in 
Paris,  seven  of  very  great,  and  two  of  extraordinary  heat.  The  mean 
of  the  summer  was  very  high,  69|-°  Fahr.  Crops  were  destroyed,  and 
forests  burned,  in  Sweden  and  Denmark. 

The  highest  temperatures  observed  were — 

Maastricht,  August  2 101'8  degrees. 

Spinal,  July  1 ....       97'7        " 

Paris,  August  1 j 97'2        " 

Metz,  August  3 97-0       " 

Strasbourg 93'6       " 

1834. — This  year,  though  not  remarkable  for  any  very  great  heat,  is 
noticeable  for  the  very  high  mean  temperature  of  the  spring  and  sum- 
mer throughout  France.  Vegetation  was  very  forward,  and  there  fell, 
in  many  places,  rain  distributed  in  such  a  manner  as  to  be  most  favor- 
able to  the  crops.  In  Paris  there  were  forty -three  days  of  great,  and 
three  days  of  very  great  heat. 

The  mean  average  of  the  summer,  69°,  is  the  highest  of  the  century, 
next  to  1826, 1842,  and  1846.  The  drought  was  very  great  in  August. 

The  maxima  temperatures  of  1834  are  thus  distributed : 

Avignon,  July  14 95'0  degrees. 

Geneva,  July  18 93'9 

Liege 92-3 

Metz,  July  12 91 '4 

Strasbourg 91 '0 

Paris,  July  12  and  18 90 '7 

In  the  south,  the  temperature  was  lowered  by  plentiful  rains,  and  was 
very  mild.  Burgundy  was  this  year  celebrated  for  the  superior  quali- 
ty of  its  wine,  though  the  quantity  ran  very  short.  Such  was  also  the 


222  THE  ATMOSPHERE. 

case  in  the  Bordeaux  district.  The  harvest  was  almost  universally 
good  in  France. 

1835, The  summer  of  this  year  was  memorable  for  the  stormy  na- 
ture of  the  month  of  June  and  the  early  part  of  July,  and  for  the  num- 
ber of  fatal  accidents  caused  by  the  heat  in  the  south  of  France.  In 
Denmark,  Eussia,  and  Spain,  the  temperature  also  produced  some  re- 
markable effects. 

The  drought  in  the  month  of  August  was  intense ;  the  Seine  fell  about 
ten  inches  below  the  low-water  mark  of  1719.  There  was  an  average 
crop  of  wine  in  the  south  of  France,  the  quality  being  fairly  good.  The 
vintage  did  not  begin  in  Burgundy  till  the  6th  of  October.  The  corn 
harvest  was  bad. 

1842. — The  summer  of  this  year  was  the  hottest  during  the  first  half 
of  this  century,  especially  in  Paris  and  the  north. 

In  Paris  there  were  of 

Great  heat 51  days. 

Very  great 11    " 

Extraordinary 4    ' 

The  mean  temperature  of  the  season  in  Paris  was  69° -4. 
The  following  is  a  list  of  the  highest  temperatures  recorded : 

Paris,  August  18 99'0  degrees. 

Agen,July4 98'6       " 

Bordeaux,  July  16 94'6       "• 

Toulouse,  July  17 93'9        " 

Many  accidents  caused  by  the  heat  were  registered ;  wheels  of  sev- 
eral mail-carts  took  fire.  At  Badajoz,  in  Spain,  three  laborers  died  of 
the  heat  on  the  28th  of  June,  and  a  lady  expired  from  its  effects  in  a 
diligence.  At  Cordova  several  reapers  fell  down  asphyxiated,  and  fre- 
quent cases  of  madness  were  attributed  to  the  same  cause. 

In  Burgundy  the  vintage  began  on  the  21st  of  September,  the  crop 
being  abundant  and  the  quality  good.  The  grain  crop  was  below  the 
average. 

1846. — The  temperature  this  summer  was  very  remarkable,  and  there 
were  periods  of  intense  heat  in  France,  Belgium,  and  England.  In 
Paris  there  were  of 

Great  heat 48  days. 

Very  great 9    " 

Extraordinary 2    " 

The  mean  summer  temperature  was  69°'2. 


HIGH  TEMPERATURES.  223 

The  maxima  of  this  year  are  as  follows : 

Toulouse,  July  7 104 '0  degrees. 

Quimper,  June  19 100-4  " 

Rouen,  July  5... 98'2  " 

Paris,  July  5 97'7  " 

Orange,  July  13 97'7  " 

Angers,  July  29 95'0 

Metz,  August  1 94-6  " 

Accidents  occurred  in  Brittany.  At  the  Pont-de-Croix  fair  several 
persons  had  fits,  occasioned  by  the  heat ;  at  Benzec,  a  little  girl,  left  in 
the  sun,  was  found  dead  a  few  minutes  afterward.  The  temperature 
during  June  was  also  very  high  at  Toulouse,  Toulon,  and  Bordeaux. 
In  the  Landes,  farmers  obtained  a  second  crop  of  rye.  Near  Niort, 
early  in  July,  three  laborers  died  while  at  work. 

The  vintage  in  Burgundy  began  on  the  14th  of  September,  the  qual- 
ity being  exceptionally  good,  though  there  was  only  a  half  crop.  The 
corn  harvest,  too,  was  much  below  the  average. 

1849. — The  heat  was  very  great  in  the  south  of  France,  and  the  max- 
imum at  Orange  is  the  highest  temperature  in  the  shade  yet  recorded 
in  France. 

The  table  gives  the  following  figures: 

Orange,  July  9 106'5  degrees. 

Toulouse,  June  23 99'7       " 

Bordeaux,  July  7 94'3       " 

Gand 93'9 

Metz,  JulyS 92'5       " 

1852. — This  was  a  remarkable  summer  in  Eussia,  England,  Holland, 
Belgium,  and  France.  There  were  in  Paris  of 

Great  heat 30  days. 

Very  great : 6     " 

Extraordinary 1     " 

The  summer  mean  in  Paris  was  67°.  The  mean  of  July  was  72^°. 
There  was  an  unusual  continuance  of  great  heat:  July  9th,  880>0;  the 
10th,  92° -3 ;  the  llth,  87°'8 ;  the  12th,  90°'5 ;  the  13th,  92°'8 ;  the  14th, 
93°-6;  the  15th,  93°'6;  the  16th,  95°'2. 

The  highest  temperatures  throughout  Europe  were — 

Constantinople,  July  27 101 '3  degrees. 

Rouen,  July  5 97'0       " 

Versailles,  July  16 96'3 


224  THE  ATMOSPHERE. 

Orange,  August  25 95'5  degrees. 

Dunkerque,  July  7 96 '3 

Paris,  July  16 95'2        " 

Venders,  July  18 95'2        " 

London,  July  12 95'0 

At  Amsterdam,  a  thermometer  rose,  on  July  the  12th,  to  102°-2.  At 
Alphen,  near  Leyden,  two  peasants,  asphyxiated  by  the  heat,  were  found 
dead  in  a  field ;  at  Alkenaer  an  engine-driver  became  insane,  after  con- 
gestion of  the  brain  produced  by  sun-stroke.  In  the  centre  of  France 
the  thermometer  stood  for  more  than  10  days  at  over  86°.  Many  do- 
mestic animals  perished  from  the  heat.  At  Thourotte,  in  Belgium, 
there  fell  a  disastrous  hailstorm  on  the  llth  of  August:  many  of  the 
hailstones  weighed  from  two  to  three  ounces,  and  were  from  two  to 
three  inches  in  diameter. 

In  France  the  harvest  was  mostly  over  by  the  middle  of  July,  and 
was  an  average.  On  the  other  hand,  the  vintage  did  not  begin  till  the 
early  part  of  October,  and  the  wine  crop  was  small  in  most  vineyards, 
and  of  inferior  quality. 

1857. — -This  summer  was  hotter  than  usual  in  France,  and  the  months 
of  July  and  August  were  nearly  everywhere  distinguished  for  extreme 
heat.  The  highest  temperatures  observed  were — 

Montpellier,  July  29 101 '5  degrees. 

Orange,  July  18 IGO'9 

Les  Mesneux,  August  4 98'6  " 

Toulouse,  July  27 98'2  " 

Clermont,  July  14  and  15,  and  August  3 98'2  " 

Blois,  in  August 97'7  " 

Paris,  August  4 97'2  " 

Metz 96-1 

There  were  three  distinct  streams  of  summer  heat.  The  first,  on  the 
27th  of  June,  passed  over  the  highest  and  the  most  southerly  stations 
in  France,  and  reached,  on  the  28th,  the  northern  frontier;  the  second 
extended  over  the  north-west,  from  the  14th  fo  the  16th  of  July :  the 
3d,  and  the  most  intense,  moved  slowly  and  in  the  same  direction,  trav- 
eled from  south  to  north  in  the  interval  between  July  the  27th  and 
August  the  4th.  The  drought  this  summer  was  very  great  through- 
out nearly  the  whole  of  France ;  fortunately,  in  the  middle  of  August, 
some  rain  fell. 

In  Burgundy  the  vintage  began  on  the  16th  of  September,  and  the 


HIGH  TEMPERATURES.  225 

crop  was  passable  as  to  quantity  and  good  as  to  quality.     The  corn 
crops  were,  generally  speaking,  up  to  the  average. 

1858. — This  summer  was  remarkable  for  great  drought,  and  pro- 
longed rather  than  intense  heat,  in  England,  Belgium,  the  centre  and  a 
part  of  the  south  of  France,  and  Algeria.  In  the  north  the  heat  was 
less  than  in  1857,  but  greater  in  the  south.  The  maxima  tempera- 
tures were — 

Montpellier,  June  20 100-9  degrees. 

Orange,  July  19 100'9 

Vendome,  June  15 97'0 

Tours,  June 96'8 

Clermont 96'4 

Lille,  June  15 95'9 

London,  June  16 94 '8 

Paris,  June3 89'6 

The  drought  was  very  great  throughout  nearly  all  France  in  tne 
spring  and  part  of  the  summer,  and  was  very  inimical  to  the  rearing  of 
stock ;  during  June  the  sky  was  remarkably  clear,  but  in  July  and  Au- 
gust some  rain  fell,  at  least  in  the  north,  so  that  the  meadows  that  had 
been  scorched  up,  owing  to  a  want  of  moisture  dating  from  the  year  be- 
fore, partially  recovered  themselves.  The  harvest,  which  terminated  on 
the  1st  of  July  in  the  south,  and  the  1st  of  August  in  the  north,  was  an 
average  crop  in  respect  to  quantity,  and  a  rather  more  than  average  one 
in  respect  to  quality.  The  vintage,  begun  in  Burgundy  on  the  18th  of 
September,  yielded  a  remarkable  crop,  both  in  respect  to  quantity  and 
quality. 

During  recent  years  I  must  mention  1865  and  1868  as  having  been 
marked  by  a  long  series  of  hot  days.  The  former,  as  is  well  known, 
was  very  favorable  for  the  vintage. 

1865. — The  mean  monthly  temperatures  at  the  Paris  Observatory 
were — 


January 38'5  degrees. 

February 36'1 

March 36'0 

April 60-4 

May 61-3 

June .  64-4 


July 67'8  degrees. 

August 63-9       " 

September 66'6       " 

October 54'0       " 

November 46'4       " 

December 36'1       " 


The  extreme  heat  in  Paris  was  91°'9  on  the  6th  of  July.  The  aver- 
age of  the  three  summer  months  was  65°'3.  Adding  to  them  Septem- 
ber, the  average  of  the  four  months  was  65°'5;  an  average  that  rarely 

15 


226  THE  ATMOSPHERE. 

lasts  so  long.  The  mean  of  the  year  was  52°-o,  being  l°-2  above  the 
average.  The  month  of  January  was  relatively  warm.  In  April,  after 
the  4th,  the  weather  was  exceptionally  fine,  and  the  thermometer  read- 
ings were  very  high :  from  the  8th  the  temperature  was  that  of  June. 
In  May  and  Jane  thte  temperatures  were  above  their  normal  points. 
July  and  August  were  cold.  In  September  the  temperature  was  higher 
than  in  August.  October  and  November  were  warm.  The  highest 
temperatures  were — 

Nimes,  July  5 100-2  degrees; 

Nice,  July  10 95'5       " 

Perpignan,  July  4 95'4       " 

Aix,  Augustus 94-5       " 

Montpellier,  July  26 93'2       " 

1868. — The  mean  monthly  temperatures  at  the  Paris  Observatory 
were — 

January 32'0  degrees. 


February 41'7 

March 44'6 

April 50-9 

May 64-2 

June 64-4 


July 70'2  degrees. 

August 65'7       " 

September 63'7       " 

October 50-9       " 

November 40'8       " 

December....  .  47 '5       " 


The  maximum  temperature  in  Paris  was  930-2  on  the  22d  of  July.  The 
average  of  the  three  summer  months  was  66°'9.  This  summer  is  nota- 
ble in  the  annals  of  meteorology  for  its  thermometrical  elevation,  and 
its  combination  of  circumstances  favorable  to  the  crops,  both  as  to  their 
quantity  and  quality.  The  averages  of  the  temperatures  of  May,  June, 
and  July  were  very  high  in  the  south.  Thus  at  Tours  the  average  of 
May  was  65°'l;  that  of  June,  67°'6 ;  that  of  July,  71°'2.  The  highest 
temperatures  observed  in  France  are  appended  : 

Nimes,  July  20 106'5  degrees. 

Perpignan,  July  25 99'0 

Draguignan,  July  24 : 98'4  " 

Montauban,  July  20 98'1  " 

Toulouse,  July  19 95'0  " 

Montpellier,  July  20 94'3  " 

Aix,  July  20 93'2  " 

The  temperature  rose  higher  in  1859,  without  giving  so  high  an  aver- 
age. This  latter  was  due  less  to  the  height  of  the  diurnal  maxima  than 
to  that  of  the  nocturnal  minima.  In  fact,  notwithstanding  the  almost 
uninterrupted  serenity  of  the  nights,  the  cold  caused  by  nocturnal  radi- 
ation was  at  no  time  very  remarkable.  Nearly  every  morning  before 


HIGH  TEMPERATURES. 


227 


sunrise  a  slight  fog,  indicating  a  somewhat  elevated  hygrometrical  con- 
dition, covered  the  soil,  moistening  the  plants,  and  modifying  the  effects 
of  the  great  heat  during  the  day.  The  vapor  of  "water  prevents  the 
radiation  of  the  obscure  heat;  the  air  which  was  lying  over  our  part 
of  the  country,  and  the  somewhat  elevated  hygrometrical  condition  of 
which  increased  the  transparency  for  the  stellar  light,  nullified  the  ef- 
fects of  nocturnal  radiation,  which  is  so  potent  even  in  the  tropical  re- 
gions when  it  has  only  to  traverse  an  air  devoid  of  moisture. 

This  remarkable  summer  affected  the  temperature  of  the  soil  to 
the  depth  of  more  than  a  yard.  During  the  summers  of  1864, 1865, 
1866,  and  1867,  the  heat  at  the  depth  of  39  inches  was  57°'7,  58°'5, 
57°*2,  and  57°*6.  In  1868  it  was  60°'6,  nearly  61°*0. 

Such  are  the  memorable  summers  of  the  present  century.  The  follow- 
ing are  the  highest  temperatures  of  the  air  (in  the  shade,  and  to  the 
north)  observed  in  France  since  they  have  been  truthfully  ascertained 
by  the  thermometer.  I  have  recorded  all  those  which  have  reached  or 
exceeded  37°,  and  those  only,  except  in  the  case  of  Paris,  where  there 
are  several  readings.  The  towns  are  given  in  the  order  of  latitude  from 
N.  to  S. 


Places. 

Latitude. 

Lougi- 
tude. 

Elevation 
above 
the  Sea. 

Dates. 

Extreme 
Maxima. 

Saint-Omer 

Deg. 
50  '45 

Deg. 
0*05 

Feet. 

75 

August  10  1777 

Deg.. 
99*5 

50  '11 

0*54 

177 

4   1783 

99*5 

49-26 

1*15 

127 

18  1800 

100*4 

Les  Mesneux  
Metz  

49-13 
49-07 

1*37 
3*50 

279 
697 

4,  1857 
4,  1781 

99*5 
100*6 

49'00 

0*02 

469 

18  1800 

98  -6 

Paris 

48-50 

o-oo 

213 

f               26,  1765 
14,  1773 
19,  1763 
5,  6,  1705 
July        16,  1782 
8,  1793 

104*0 
102*9 

102*2 
101*7 
101*1 

48*48 

5'25 

439 

'      "          10,  1766 
August   18,  1842 
July        31,  1803 
5,  1846 
"          19,  1825 
August     4,  1857 
July        16  1782 

101*0 
99-0 
98-1 
97-7 
97-3 
97-2 
102*9 

Nancy 

48  "42 

3*51 

656 

"          26,  1782 

99*7 

Chartres 

48*27 

0'51 

518 

"         16,  1793 

100*6 

48-00 

6*26 

19 

June       19,  1846 

100-4 

48-00 

0'23 

380 

1777    and    1778 

99*5 

47'28 

2*54 

154 

July        17,  1784 

100-4 

Tours 

47'24 

1*39 

180 

August          1840 

100-4 

Nantes             

47*13 

3-53 

144 

"       18,  1800 

101-8 

47*10 

2*06 

269 

July        21,  1783 

100*6 

Seurre(C6te-d'Or)... 

47*01 

2*48 

492 

6,  1783 

102-2 

228 


THE  ATMOSPHERE. 


Places. 

Latitude. 

Longi- 
tude. 

Elevation 
above 
the  Sea. 

Dates. 

Extreme 
Maxima. 

46-47 

Deg. 
3-42 

Feet. 
492 

Julv,             1787 

Deg. 
99-5 

46-27 

3-30 

265 

21,  1777 

101-8 

La  Rochelle  

46-09 

3-30 

82 

4,  5,  1836 

102-2 

Saint-Jean  d'Angely. 

45-57 
45-50 

2-52 
1-05 

78 
941 

1787 
'    23-25,  1800 

99-5 
99-5 

Valence             

44-56 

2-33 

419 

'          11,  1793 

104-0 

44-50 

2-55 

59 

August     6,  1800 

101-8 

Joyeuse  (Ardeche).... 

44-32 
44-12 

2-00 
1-43 

482 
141 

June       23,  1822 
July         4,  1842 

99-1 
98-6 

44-08 

2'28 

150 

9,  1849 

106-5 

43-57  N. 

2-28 

118 

jAugust  14,  1802 

-  100-6 

Nimes    

43-51 

2-01 

374 

(      "       16,  1803 
July        20,  1868 

) 
106-5 

43-49 

3-35 

1312 

"          18,  1782 

101-8 

Aries 

43-41 

2-18 

55 

August  20,  1806 

99-5 

43-37 

0-54 

649 

(July  30,  31,  1753 

99-9 

43-37 

1-32 

98 

(    "            7,  1846 
"          29,  1857 

104-0 
101-5 

Be'ziers 

43-21 

0-52 

252 

"                1847 

98-6 

Soreze             

43-19 

0-13 

1640 

"          12,  1824 

99-5 

Pau  

43-18 

2-43 

672 

August     4,  1838 

101-8 

Perpignan  

41-42 

0-34 

137 

Julv        29,  1857 

101-5 

The  greatest  beat  in  the  shade  and  in  the  north  is  106^°  for  France 
(Orange  July  9th,  1849,  and  Nimes  July  20th,  1868) ;  96°  for  Great 
Britain ;  102°  for  Holland  and  Belgium ;  99£°  for  Denmark,  Sweden, 
and  Norway ;  102°  for  Russia ;  103°  for  Germany ;  105°  for  Greece ; 
104°  for  Italy ;  102°  for  Spain  and  Portugal.  In  non-European  coun- 
tries, the  highest  temperatures,  as  given  by  Arago,  are  as  follows : 

Tunis 112-5  degrees. 

Manilla 113'5 

Nubia 115-2  " 

Am-Dize  (Egypt) 116'1  " 

Esne'  (Africa) 117"3  " 

Bagdad  (Asia) 120'0  " 

Near  Suez  (French  Expedition  to  Egypt) 126'5  " 

Near  Port  Macquarie  (Archipelago) 129'0  " 

NearSyene  (Africa) 129'2 

Murzouk  (Africa) 133-4  " 

These  are  the  maxima  of  the  temperatures  of  the  air  in  the  shade. 

In  presence  of  such  elevations  of  temperature,  it  may  be  asked  to 
what  point  human  organism  can  support  it  without  incurring  the  dan- 
ger of  sudden  death.  The  mean  temperature  of  the  human--  body  is 
about  96°  (it  is  easily  ascertained  by  placing  the  bulb  of  a  thermometer 
under  the  tongue).  That  of  birds  is  higher,  and  reaches  111°  with  cer- 
tain kinds.  That  of  fish  is  lower,  and  about  37°. 


AUTUMN— WINTER.  £29 


CHAPTER  V. 

AUTUMN — WINTER  :  WINTER  LANDSCAPES — COLD — SNOW — ICE — HOAR- 
FROST, RIME,  ETC.  —  REMARKABLE  WINTERS  —  THE  LOWEST  KNOWN 
TEMPERATURES. 

TAKE,  in  the  first  place,  this  winter  landscape  which  is  represented  in 
the  preceding  page.  It  is  the  same  as  that  which  we  saw,  full  of  color 
and  movement,  on  a  fine  summer's  day.  It  is  now  transformed  be- 
neath the  gray  and  sombre  sky  of  winter.  The  green  foliage  has  dis- 
appeared from  the  trees,  the  meadow  is  covered  with  a  pall  of  snow,  the 
rivulet  is  frozen  over,  and  the  laborer's  cottage  seems  as  lifeless  as  Na- 
ture herself.  With  the  progressive  decline  of  temperature  the  ther- 
mometer has  fallen  to  32°,  a  remarkable  point,  at  which  water  ceases  to 
preserve  its  liquid  condition  and  becomes  solid.  It  then  may  assume 
various  forms,  becoming  either  massive  in  the  shape  of  ice,  light  in  the 
shape  of  hoar-frost,  or  falling  slowly  as  snow-flakes.  It  is,  as  a  rule,  in 
this  latter  form  that  winter  begins  to  manifest  itself,  for  snow  is  pro- 
duced as  soon  as  the  temperature  is  at  or  about  32°.  If  this  tempera- 
ture extends  from  the  clouds  to  the  surface  of  the  earth,  the  water 
reaches  the  ground  as  snow.  If  snow  in  falling  has  only  a  thin  stratum 
of  air  above  32°  to  traverse,  and  if  it  be  abundant,  it  still  reaches  the 
ground  and  preserves  its  consistency.  This  occurs  sometimes  in  sum- 
mer. 

Snow,  in  covering  the  earth  as  a  carpet,  forms  at  once  a  covering  and 
a  screen ;  a  screen,  because,  possessing  but  little  conducting  power,  it 
obstructs  the  passage  of  heat  from  the  earth,  and  thus  prevents  the 
earth  from  becoming  as  cold  as  the  air.  Snow  also  adds  its  influence 
in  favor  of  the  fertilizing  of  the  soil.  Like  rain  and  mists,  it  contains  a 
considerable  proportion  of  ammonia,  which  exists  in  a  volatile  state  in 
the  atmosphere,  and  which  it  conveys  to  the  soil,  afterward  preventing 
it  from  becoming  volatile  again,  as  is  the  case  after  rain,  especially  after 
warm  rain. 

In  the  origin — that  is  to  say,  in  the  frozen  clouds  high  up  in  the  at- 
mosphere— the  snow  appears  to  be  formed  of  very  slender  fibres  of  ice. 
When  the  small  drops  of  water,  which  form  mists  and  ordinary  clouds, 


230  TEE  ATMOSPHERE. 

become  congealed,  it  is  probable  that  these  drops  do  not  preserve  their 
spheroidal  shape,  but  that  they  fall  an  instant  and  take  the  shape  of  a 
filament  which  freezes  concurrently  with  its  physical  transformation. 
By  virtue  of  the  laws  of  crystallization,  these  small  filaments  of  ice  be- 
come cohesive  at  angles  of  60°,  and  form  the  figures  which,  though  so 
numerous,  still  appertain  to  the  same  geometrical  order. 

Glaisher,  in  his  ascent  of  June  26,  1863,  encountered  at  13,000  feet 
an  immense  cloud  of  snow,  extending  to  a  thickness  of  nearly  one  mile. 
It  was  a  truly  wonderful  sight.  This  snow  was  composed  entirely 
of  small  and  perfectly -formed  crystals,  of  an  extreme  delicacy.  The 
points  were  visible,  separate  from  each  other,  following  two  systems  of 
crystallization;  for  the  angular  intervals  were  some  at  60°,  and  others 
at  90°. 

The  construction  of  snow-flakes  has  long  attracted  the  attention  of 
observers.  Kepler  speaks  of  their  structure  with  admiration,  and  other 
natural  philosophers  have  endeavored  to  determine  their  cause ;  but  it 
is  only  since  the  laws  of  crystallization  in  general  have  been  ascertain- 
ed that  it  has  been  possible  to  throw  any  light  upon  this  subject. 

In  a  circle,  of  all  the  polygons  which  can  be  inscribed,  there  is  but 
one  whose  sides  are  equal  to  its  radius ;  that  is,  the  regular  hexagon,  or 
figure  with  six  sides.  This  hexagonal  figure  is  traced  upon  the  flowers 
of  the  field,  and  we  meet  with  it  also  in  the  crystallization  of  ice  and 
snow,  in  the  analysis  of  all  the  forms  presented  to  our  notice.  The 
tendency  of  ice  to  take  a  crystalline  shape  is  made  evident  by  the  fern- 
like  leaves  noticeable  on  window-panes  during  winter,  when  water  be- 
comes congealed  upon  them. 

The  examination  of  the  figures  of  snow  leads  to  impressions  not  less 
marked  as  to  the  existence  of  geometry,  Number  and  Beauty,  in  the 
works  of  nature.  It  is  not  merely  a  few  ice-flowers,  such  as  the  above, 
which  have  been  remarked  and  designed  in  the  slender  snow-flakes,  but 
there  are  many  hundred  different  kinds,  all  constructed  upon  the  same 
fundamental  angle  of  60°. 

The  snow  sometimes  falls  in  such  compact  flakes  that  behind  the  first 
planes  it  forms  a  white,  cloudy  veil,  which  hides  the  landscape.  These 
heavy  falls  of  snow  are  mostly  met  with  upon  the  lofty  table-lands  of 
Asia  or  the  Andes,  where  the  caravans  have  often  to  encounter  them. 
The  routes  soon  become  concealed  beneath  the  pall  that  covers  them ; 
it  becomes  difficult  to  find  one's  way;  and  just  as,  in  the  rarer  falls  of 
snow  in  our  countries,  travelers  wander  over  St.  Bernard,  and  even 


Fig.  62. — Snow  Crystals. 


AUTUMN— WINTER.  233 

over  the  plains  of  France,  to  fall  at  last  into  the  sleep  of  death,  so  in  the 
more  frequent  downfalls  in  these  regions  does  the  traveler  come  to  a 
halt,  having  lost  his  way,  sinking  into  the  ravines  if  he  attempts  to  ad- 
vance, falling  into  a  lethargy  if  he  remains  to  rest,  and  often  finding  no 
escape  but  death. 

A  very  beautiful  form  of  crystallization  appears  in  winter,  autumn, 
and  spring  mornings  around  branches  of  trees,  upon  the  twigs  of  plants, 
and  grass,  when  the  temperature  of  the  air  is  below  32°.  This  is  hoar- 
frost, which  might  also  be  termed  an  icy  dew,  the  embroidery-like  work 
of  which,  often  so  beautiful,  gives  our  winter  landscape  that  special 
mixture  of  severity  and  soft  melancholy  by  which  it  is  characterized. 
The  hoar-frost  is  chiefly  formed  upon  cold,  misty  mornings,  and  it  is 
often  late  in  the  afternoon  before  the  sun  has  melted  these  filmy  vege- 
table stalactites. 

When  the  temperature  of  the  air  has  been  for  some  time  below  32°, 
still  waters  freeze  upon  their  surface.  A  small  wrinkle  first  deadens 
the  surface,  and  forms  a  thin  layer,  which  gets  thicker  and  whitens,  if 
the  frost  continues.  The  theory  explains  itself  by  the  equilibrium  of 
the  strata  of  water  of  different  temperatures  and  different  densities. 

If  liquids  of  different  densities  and  without  any  chemical  affinity  are 
thrown  into  a  vessel  of  water,  the  heaviest  will  finally  fall  to  the  bot- 
tom, and  the  lightest  will  remain  upon  the  surface.  All  bodies  increase 
in  density  as  their  temperature  diminishes.  Water  alone,  to  a  certain 
very  limited  extent  of  the  thermometrical  scale,  offers  a  singular  excep- 
tion to  this  rule.  Let  us  take  water  at  50°  Fahr. :  it  increases  in  densi- 
ty as  the  temperature  decreases  to  about  39°. 

Eivers  do  not  begin  to  freeze  until  there  is  a  freezing  temperature  of 
about  20°.  Large  streams,  to  be  frozen  over  from  bank  to  bank,  must 
be  exposed  to  a  temperature  still  lower,  in  proportion  to  their  rapidity. 
As  the  severity  of  the  frost  becomes  prolonged,  the  thickness  of  the 
coat  of  ice  formed  increases,  and  becomes  capable  of  bearing  men  and 
carts  upon  it;  so  much  so,  that  the  fact  of  the  weight  that  can  be  placed 
upon  it  is  the  proof,  almost  the  measure,  of  the  intensity  of  the  cold. 
It  is,  therefore,  interesting  to  know  what  thickness  of  ice  will  support  a 
given  weight.  It  has  been  ascertained  that  ice  two  inches  in  thickness 
will  bear  the  weight  of  a  man,  four  inches  in  thickness  that  of  a  person 
on  horseback;  that  when  the  ice  is  six  inches  thick  it  will  bear  eight- 
pounders  placed  upon  sledges,  and  that  at  eight  inches  field  artillery 
may  cross  it  in  safety.  The  heaviest  of  carriages,  an  army,  or  a  large 


234  THE  ATMOSPHERE. 

crowd,  are  in  no  danger  when  standing  upon  ice  eleven  or  twelve  inch' 
es  thick. 

In  very  severe  Eussian  winters  the  ice  in  the  rivers  is  more  than  one 
yard  thick ;  but  in  France  it  has  never  exceeded  more  than  about  two 
feet  Its  power  of  resistance  is  so  great  that,  in  1740,  a  large  palace  of 
ice  was  constructed  at  St.  Petersburg,  55|-  feet  long,  17  feet  wide,  and 
21  feet  high,  the  weight  of  the  top  and  of  the  higher  parts  of  the  edifice 
being  readily  supported  by  the  foundations.  In  front  of  the  building 
were  placed  six  guns  in  ice,  with  their  carriages  made  of  the  same  ma- 
terial. They  were  made  to  fire  ball ;  and  each  piece  pierced,  at  a  dis- 
tance of  sixty  yards,  a  plank  two  inches  in  thickness.  The  guns  were 
not  more  than  four  inches  thick;  they  were  loaded  with  a  quarter  of  a 
pound  of  powder,  and  not  one  of  them  burst.  The  Neva  supplied  the 
materials  for  this  singular  edifice. 

I  have  said  that  water  when  congealed  increases  in  volume.  One 
consequence  and  one  proof  of  this  expansion  is  the  bursting  of  the  ves- 
sels containing  it— a  fact  which  occurs  all  the  more  readily  when  the 
process  of  freezing  is  rapid  and  the  vessel  narrow  in  the  neck. 

I  will  complete  this  chapter  by  a  notice  of  some  of  the  hardest  winters 
-upon  record — considering  those  as  hard  winters  in  which  the  cold  has 
been  of  sufficient  length  and  severity  to  freeze  certain  sections  of  large 
rivers,  such  as  the  Seine,  the  Saorie,  and  the  Ehine — to  congeal  wine, 
to  destroy  the  tissues  of  certain  trees,  and  to  be  followed  by  very  grave 
consequences  for  both  the  vegetable  as  for  the  animal  world. 

The  following  among  the  remarkable  winters  are  the  severest  during 
the  last  hundred  years.  Let  me,  in  the  first  place,  mention  that  the 
hardest  winters  of  past  centuries  were  those  of  1544,  1608,  and  1709,  in 
which  latter  year  the  thermometer  at  the  Paris  Observatory  fell  as  low 
as  —  9°'6  Fahr.  The  winter  of  1776  next  comes  as  an  exceptionally 
cold  one.  The  Tiber,  the  Khine,  the  Seine,  and  even  the  Ehone,  rapid 
as  it  is,  were  nearly  entirely  frozen  over. 

After  1776,  we  come  to  the  winter  of  1788-1789,  precursor  of  the 
Eevolution.  This  was  one  of  the  severest  and  longest  winters  that  have 
ever  prevailed  in  Europe.  In  Paris  the  cold  commenced  on  the  25th 
of  November,  and  lasted,  with  the  exception  of  Christmas-day,  when  it 
did  not  freeze,  for  fifty  consecutive  days.  The  thaw  began  on  the  13th 
of  January,  and  the  snow  was  found  to  be  twenty-six  inches  deep.  In 
the  great  canal  at  Versailles,  in  the  ponds  and  in  several  streams,  the 
ice  was-  two  feet  thick.  The  water  also  froze  in  several  very  deep  wells, 


A  TTTUMN—  WINTER. 


235 


and  wine  became  congealed  in  cellars.  The  Seine  began  to  freeze  as 
early  as  November  26th  (1788),  and  for  several  days  its  course  was  im- 
peded, the  breaking  up  of  the  ice  not  taking  place  until  the  20th  of 
January.  The  lowest  temperature  observed  at  Paris  was  — 70-2  Fahr., 
on  the  31st  of  December.  The  frost  was  equally  severe  in  other  parts  of 
France  and  throughout  Europe.  The  Rhone  was  quite  frozen  over  at 
Lyons,  the  Garonne  at  Toulouse;  and  at  Marseilles  the  sides  of  the  docks 


Fig.  53.-Winter.— The  Seine  full  of  floating  ice. 

were  covered  with  ice.  Upon  the  shores  of  the  Atlantic  the  sea  was 
frozen  to  a  distance  of  several  leagues.  The  ice  upon  the  Rhine  was  so 
thick  that  loaded  wagons  were  able  to  cross  it.  The  Elbe  was  covered 
with  ice,  and  also  bore  up  heavy  carts.  The  harbor  at  Ostend  was 
frozen  so  hard  that  people  could  cross 'it  on  horseback;  the  sea  was 
congealed  to  a  distance  of  four  leagues  from  the  exterior  fortifications, 
and  no  vessel  could  approach  the  harbor.  The  Thames  was  frozen  as 
low  as  Gravesend,  and  during  the. Christmas  holidays  and  the  early 


236  THE  ATMOSPHERE. 

part  of  January  the  stream  in  the  neighborhood  of  London  was  covered 
with  shops. 

The  following  are  the  lowest  temperatures  that  were  noted  in  differ- 
ent places : 

Bale  (Suisse),  December  18 -35'5  degrees. 

Bremen  (Germany),  December  16 -32-1 

St.  Alban's,  December  31 -28 '8 

Warsaw  (Poland),  December  18 -26 -6 

Dresden  (Germany),  December  17 -25 "8 

Eosberg  (Norway),  December  29 -24'3 

St.  Petersburg,  December  12 -28'1 

Berlin  (Prussia),  December  28 -19 '8 

Strasbourg,  December  31 —16*3 

Tours,  December  31 -IS'O       " 

Lons-le-Saulnier,  December  31 -11 '2       " 

Troyes,  December  31 -10'8       " 

Orleans,  December  31 —  8 '5 

Lyons,  December  31 -  7'4       " 

Eouen,  December  30 -7-2       " 

Paris,  December  31 —  7'2       " 

Grenoble,  December  31 —  6.2       " 

Angouleme,  December  31 —  1'7 

Marseilles,  December  31 +  1'4       " 

The  cold  of  this  winter  was  very  fatal  to  men  and  animals,  and  in- 
jurious to  vegetables.  In  the  Toulouse  district  the  bread  was  nearly 
everywhere  frozen,  and  it  was  impossible  to  cut  it  until  it  had  been  laid 
before  the  fire.  Several  travelers  perished  in  the  snow ;  at  Lemberg, 
in  Galicia,  thirty-seven  persons  were  found  dead  in  three  days  toward 
the  end  of  December.  The  birds  that  belong  to  the  extreme  north 
were  seen  in  several  parts  of  France.  Fish  were  killed  in  nearly  all 
the  ponds  by  the  great  depth  to  which  the  ice  penetrated. 

1794-95. — This  was  a  remarkably  long  and  severe  winter  throughout 
Europe.  In  Paris  there  were  forty-two  consecutive  days'  frost;  and 
January  25th  (1795)  was  the  coldest  day  ever  known,  the  thermometer 
falling  to  — 100<3,  or  42°'3  below  the  freezing-point  of  water.  In  Lon- 
don the  minimum  temperature,  8°'l,  occurred  upon  the  same  day ;  and 
at  midnight,  on  the  banks  of  the  Rhone,  near  Geneva,  it  was  6°'8. 
The  Maine,  the  Scheldt,  the  Ehine,  and  the  Seine  were  so  frozen  over, 
that  carriages  and  army  corps  crossed  them  in  several  places.  The 
Thames  was  frozen  over  in  the  beginning  of  January,  near  Whitehall, 
in  spite  of  the  height  of  the  tide.  Pichegru,  then  in  the  north  of  Hol- 
land, sent  detachments  of  cavalry  and  infantry  about  the  20th  of  Jan- 


AUTUMN-WINTER.  237 

uary,  with  orders  to  the  former  to  cross  the  Texel,  and  to  capture  the 
enemy's  vessels  caught  at  anchor  by  the  frost.  The  French  horsemen 
crossed  the  plains  of  ice  at  full  gallop,  approached  the  vessels,  called  on 
them  to  surrender,  captured  them  without  a  struggle,  and  took  the  crews 
prisoners. 

1798-99. — This  was  a  very  cold  winter  all  over  Europe.  In  Paris 
there  were  thirty-two  consecutive  days'  frost,  and  the  Seine  was  com- 
pletely frozen  from  the  29th  of  December  to  the  19th  of  January,  from 
the  Pont  de  la  Tournelle  to  beyond  the  Pont  Royal,  but  not  sufficient- 
ly so  to  admit  of  its  being  crossed  on  foot.  The  lowest  temperature 
remarked  was  +0°'3,  or  31°'7  below  the  freezing-point  of  water  in 
Fahrenheit's  scale,  on  December  10th,  1798.  An  Alpine  eagle  was 
shot  at  Chaillot.  The  Meuse,  the  Elbe,  and  the  Rhine  were  frozen 
more  completely  than  the  Seine.  Carriages  crossed  the  Meuse ;  at  the 
Hague  and  at  Rotterdam  fairs  were  held  upon  the  stream.  A  regiment 
of  dragoons,  starting  from  Mayence,  crossed  the  Rhine  upon  the  ice  in- 
stead of  by  the  bridge  at  Cassel,  which  it  had  been  found  necessary  to 
raise. 

1812-13. — This  winter  will  ever  be  remembered  for  the  terrible  dis- 
asters which  attended  the  retreat  of  the  French  army  through  Russia, 
after  the  capture  and  conflagration  of  Moscow.  The  frost  set  in  early 
all  over  Europe.  The  retreat  of  the  army  began  on  the  18th  of  No- 
vember; Napoleon  left  the  capital  of  the  Muscovite  Empire  on  the 
19th,  and  the  evacuation  of  the  city  was  complete  on  the  23d.  The 
army  marched  toward  Smolensk,  the  snow  falling  without  intermis- 
sion. The  cold  became  very  intense  after  the  7th  of  November,  and 
on  the  9th  the  thermometer  marked  5°'0  (Fahr.).  On  the  17th  the  tem- 
perature fell  to  — 15°'2  (Fahr.)  according  to  Larry,  who  had  a  thermom- 
eter suspended  from  his  button-hole.  The  army  corps  commanded  by 
Ney  escaped  from  the  Russian  troops,  by  whom  it  was  surrounded,  ac- 
cording to  Arago,  by  crossing  the  Dnieper,  which  was  frozen  over,  on 
the  night  of  the  18th-19th  of  November.  The  day  before  some  Russian 
troops,  with  their  artillery,  had  crossed  the  Dwina  upon  the  ice.  The 
cold  diminished,  and  a  thaw  began  on  the  24th,  but  did  not  last;  so  that 
from  the  26th  to  the  29th,  during  the  fatal  passage  of  the  Berezina, 
the  water  contained  numerous  blocks  of  ice  without  offering  a  passage 
at  any  part  to  the  troops.  The  cold  soon  set  in  again  with  fresh  in- 
tensity; the  thermometer  fell  again  to  — 13°'0  (Fahr.)  on  the  30th  of 
November;  to  —22°  (Fahr.)  on  December  3d;  and  to  -35°  on  the  6th 


238  THE  ATMOSPHERE. 

at  Molodeczno,  the  day  after  Napoleon  left  Smorgoni,  and  published 
the  bulletin  (No.  29)  which  informed  France  of  a  part  of  the  disasters 
incurred  during  this  terrible  campaign. 

1819-20. — This  was  also  a  very  severe  winter  throughout  Europe, 
although  the  extreme  cold  did  not  last  so  long.  In  Paris  there  were 
forty-seven  days'  frost,  nineteen  of  which  were  consecutive,  from  the 
30th  of  December,  1818,  to  January  17th,  1819.  The  minimum  tem- 
perature occurred  on  the  llth  of  January,  viz.,  —14° '3.  The  Seine  was 
entirely  frozen  over  from  the  12th  to  the  19th  of  January.  The  Saone, 
the  Rhone,  the  Rhine,  the  Danube,  the  Garonne,  the  Thames,  the  La- 
goons of  Venice,  and  the  Sound,  were  so  far  frozen  that  it  was  possible 
to  walk  upon  the  ice.  The  lowest  temperatures  observed  in  different 
towns  are  as  follows : 

St.  Petersburg,  January  18 , — 25'6  degrees. 

Berlin,  January  10. : — 11'9 

Maestricht,  January  10 , —  2'7 

Strasbourg,  January  15 —  1'8 

Commercy  (Meuse),  January  12 —1*8 

Marseilles,  January  12 +  0'5 

Metz,  January  10 +  2'7 

Mons,  January  11-15 +  3'9 

Paris,  January  11 +  6'3 

In  France  the  intensity  of  the  cold  was  heralded  by  the  passage  along 
the  coast  of  the  Pas  de  Calais  of  a  great  number  of  birds  coming  from 
the  farthest  regions  of  the  north,  by  wild  swans  and  ducks  of  variegated 
plumage.  Several  travelers  perished  of  cold ;  among  others  a  farmer 
near  Arras,  a  gamekeeper  near  Nogent  (Haute  Marne),  a  man  and  a 
woman  in  the  Cote  d'Or,  two  travelers  at  Breuil,  on  the  Meuse,  a  woman 
and  a  child  on  the  road  from  Etain  to  Verdun,  six  persons  near  Chateau 
Salins  (Meurthe),  and  two  little  Savoyards  on  the  road  from  Clermont 
to  Chalons-sur-Saone.  In  the  experiments  made  at  the  Metz  School 
of  Artillery  on  the  10th  of  January,  to  ascertain  how  iron  resisted  low 
temperatures,  several  soldiers  had  their  hands  or  their  ears  frozen. 

1829-30. — This  was  the  earliest  and  longest  winter  of  the  first  part 
of  the  nineteenth  century  ;  its  duration  was  especially  injurious  to  agri- 
culture in  southern  countries.  The  cold,  without  being  extremely  rig- 
orous, extended  all  over  Europe ;  a  great  number  of  rivers  were  con- 
gealed, and  the  thaw  was  accompanied  by  disastrous  inundations ;  many 
men  and  animals  perished,  and  field  labor  was  for  a  long  time  inter- 
rupted. The  following  are  the  principal  temperatures  observed : 


A  UTUMN—  WINTER.  239 

St.  Petersburg,  December  19 — 26'5  degrees. 

Mulhouse,  February  3 — 18'6 

Bale,  February  3 — 16'6 

Nancy,  February  3 — 15'3 

Spinal,  February  3 — H'l 

Aurillac,  December  27 — 10'6 

Strasbourg,  February  3 —  lO'l 

Berlin,  December  23 —  5'8 

Metz,  January  31 —  4'9 

Pau,  December  27 +  0'5 

Paris,  January  17 +  I'O 

In  Switzerland  the  winter  was  severe  in  the  great  altitudes.  At 
Freiburg  there  were  one  hundred  and  eighteen  days'  frost,  sixty-nine 
of  which  were  consecutive,  and  the  minimum  was  — 10<3  Fahr.  In 
the  plains,  at  Yverdun,  among  other  places,  the  effects  of  radiation 
were  felt  very  intensely:  the  thermometer  fell  in  a  few  hours  from 
+  14°  to  —  4°.  The  snow  termed  polar  snow,  the  crystallization  of 
which  is  very  close,  and  which  is  peculiar  to  very  low  temperatures, 
also  fell  there. 

The  length  of  time  during  which  the  Seine  was  frozen  and  its  subse- 
quent thaw,  excited  public  curiosity  to  the  highest  degree.  The  river 
remained  frozen  from  December  28th  to  the  26th  of  January;  that  is, 
for  twenty-nine  days,  on  the  first  occasion.  It  was  frozen  over  after- 
ward from  the  oth  to  the  10th  of  February,  making  in  all  thirty-four 
days,  or  as  long  as  was  the  case  in  1763.  It  was  frozen  over  at  Havre 
from  the  27th  of  December,  and  a  fair  was  established  upon  the  ice  at 
Kouen  on  the  18th  of  January.  On  the  25th  of  January,  after  six  days' 
thaw,  the  ice  from  Corbeil  and  Melun  blocked  up  the  bridge  at  Choisy, 
forming  a  wall  sixteen  and  a  half  feet  high. 

1840-1841. — During  this  winter  there  were  fifty-nine  days'  frost, 
twenty-seven  of  them  consecutive  in  Paris.  The  cold  began  on  the  5tb 
of  December,  and  lasted,  with  an  intermission  from  the  1st  to  the  3d  of 
January,  until  the  10th  of  that  latter  month.  There  was  another  frost 
from  the  30th  of  January  to  the  10th  of  February.  On  the  3d  of  Feb- 
ruary the  thermometer  still  marked  16°'2  Fahr.  From  the  16th  of  De- 
cember the  Seine  was  full  of  blocks  of  ice,  and  one  of  the  arches  of  the 
Pont  Eoyal  was  obstructed.  Upon  the  evening  of  the  same  day  the 
current  was  stopped  at  the  Pont  d'Austerlitz,  and  was  frozen  from  Pont 
Marie  to  Charenton.  The  next  day  it  was  frozen  at  the  bridge  of  Notre- 
Dame,  and  on  the  18th  people  crossed  from  Bercy  to  the  railway  sta- 


240  THE  ATMOSPHERE. 

tion.  In  several  places  the  blocks  of  ice  forced  together  were  as  much 
as  seven  to  eight  feet  thick.  On  the  loth  of  December  the  ashes  of 
Napoleon,  brought  back  from  St.  Helena,  entered  Paris  by  the  Arc  de 
Triomphe.  The  thermometer,  in  places  exposed  to  nocturnal  radiation, 
had  that  day  marked  +  6°'8  Fahr.  An  immense  crowd,  the  National 
Guard  of  Paris  and,  its  suburbs,  and  numerous  regiments,  lined  the 
Champs  filys&s  from  the  early  morning  until  two  in  the  afternoon. 
Every  one  suffered  severely  from  the  cold.  Soldiers  and  workmen, 
hoping  to  obtain  warmth  by  drinking  brandy,  were  seized  by  the  cold, 
and  dropped  down  dead  of  congestion.  Several  persons  perished,  vic- 
tims of  their  curiosity :  having  climbed  up  into  the  trees  to  see  the  pro- 
cession, their  extremities,  benumbed  by  the  cold,  foiled  to  support  them, 
and  they  were  killed  by  the  fall.  I  append  some  of  the  temperatures 
noted  during  this  winter. 

Mount  St.  Bernard,  January  22 -9'9  degrees. 

Geneva,  January  10 +0'0       " 

Metz,  December  17 +4'5       " 

Paris,  December  17 +8'2       " 

Paris,  January  8 +84       " 

1853-54. — This  was  a  severe  winter  in  the  temperate  regions  of 
Europe.  It  lasted  from  November  to  March,  and  caused  several  rivers 
to  be  frozen  over.  The  cold  was  intense  in  many  places,  yet  it  proved 
rather  beneficial  to  agriculture1  than  otherwise. 

The  principal  temperatures  were  as  follows: 

Clermont,  December  26 —  4'0  degrees. 

Chalons  sur  Marne,  December  26 —  4'0 

Lille,  December  26 —  0'4 

Kehl,  December  26 +  0'3 

Metz,  December  27 +  0'5 

Brussels,  December  26 +  3'0 

Lyons,  December  30 +  5*7 

Paris,  December  30 +  6'8 

Bordeaux,  December  30 +14'0 

The  next  winter  was  also  severe,  especially  in  Southern  Russia,  Den- 
mark, England,  and  France,  and  was  of  unusual  length.  The  frosts 
commenced  as  early  as  October  in  the  east  of  France,  and  lasted  until 
the  28th  of  April.  The  Loire  was  blocked  with  ice  on  the  17th  of 
January,  and  its  course  was  arrested  the  next  day.  The  Seine,  though 
full  of  blocks  of  ice  on  the  19th  of  January,  was  not  frozen  over.  The 
Rhone  was  impeded  on  the  20th,  and  the  Saone  on  the  same  day.  The 


AUTUMN— WINTER.  241 

Rhine  was  completely  frozen  over  at  Manheim'ori  the  24th,  and  people 
crossed  it  on  foot.     The  appended  table  gives  the  lowest  temperatures : 

Vendome,  January  20 —  0'4  degrees. 

Clermont,  January  21 +   1'4  " 

Brussels,  February  2 +   1-9  " 

Turin,  January  24 +  2'3  " 

Metz,  January  29 +  3'2  " 

Strasbourg,  January  29 +  3'2       " 

Montpellier,  January  21 +  3'2  " 

Lille,  February  2 +  7'2  " 

Paris,  January  21 +11 '7  " 

Toulouse,  January  20 +12'7  " 

The  winter  of  1857-1858  was  the  type  of  the  average  severity  of  a 
winter  in  the  temperate  zone.  The  Seine  contained  blocks  of  ice  on  the 
5th  of  January,  and  the  small  arm  of  the  stream  by  the  Cite  was  cover- 
ed with  ice  on  the  6th.  The  Loire,  the  Cher,  the  Nie"  vre,  the  Ehone, 
the  Saone,  and  the  Dordogne  were  stopped  in  several  places.  The 
Danube  and  the  Eussian  ports  in  the  Black  Sea  were  frozen  in  January. 
The  lowest  temperatures  were : 

Le  Puy,  January  25 +  6'1  degrees. 

Clermont,  January  7 +  6'8       " 

Bourg,  January  29 +  9'5        " 

Vendome,  January  6 +12'2       " 

Lille,  January  7 -14'0       " 

Paris,  January  7 -15'8       " 

The  winter  of  1864-1865  was  more  severe.  The  Seine  was  frozen 
over  at  Paris,  and  people  crossed  it  by  the  Pont  des  Arts.  The  ex- 
treme temperatures  were : 

Haparanda,  February  7 — 28'1  degrees. 

St.  Petersburg,  February  9 — 19'8       " 

Eiga,  February  4 — 14'4        " 

Berne,  Februaiy  14 +  5'0       " 

Dunkirk,  February  15 -f-10'4       " 

Strasbourg,  February  11 +12*2       " 

Lastly,  the  winter  of  1870-1871  will  also  be  classed  among  severe 
winters,  because  of  the  extreme  cold  in  December  and  January  (not- 
withstanding the  mild  weather  of  February),  and  also  because  of  the 
fatal  influence  which  the  cold  exercised  upon  the  public  health  at  the 
close  of  the  war  with  Germany.  The  great  equatorial  current,  which 
generally  extends  to  Norway,  stopped  this  year  at  Spain  and  Portugal, 

16 


242  THE  ATMOSPHERE. 

the  prevailing  wind  being  from  the  north.  On  the  5th  of  December 
there  was  a  temperature  of  21°  Fahr.;  and  on  the  8th,  at  Montpellier, 
the  thermometer  stood  at  17° '6  Fahr.  A  second  period  of  cold  set  in 
on  the  22d  of  December,  lasting  until  the  5th  of  January.  In  Paris  the 
Seine  was  blocked  with  ice,  and  seemed  likely  to  become  frozen  over. 
On  the  24th  tkere  were  21°'6  of  frost;  and  at  Montpellier,  on  the  31st, 
28°'8.  It  is  well  known  that  many  of  the  outposts  around  Paris,  and 
several  of  the  wounded  who  had  been  lying  for  fifteen  hours  upon  the 
field,  were  found  frozen  to  death.  From  the  9th  to  the  15th  of  January 
a  third  period  of  cold  set  in,  the  thermometer  marking  +  17°'6  Fahr.  at 
Paris,  and  +8°'6  Fahr.  at  Montpellier.  The  most  curious  fact  was  that 
the  cold  was  greater  in  the  south  than  in  the  north  of  France.  At  Brus- 
sels the  minima  were  +  11°-1  in  December,  and  +  8°'2  Fahr.  in  Janu- 
ary. There  were  forty  days'  frost  at  Montpellier,  forty-two  in  Paris, 
and  forty-seven  at  Brussels,  during  these  two  months.  Finally,  the 
winter  average  (December,  January,  and  February)  is  35°'2  in  Paris, 
whereas  the  general  average  is  37°*9.  In  the  north  of  Europe  this  was 
also  a  very  hard  winter,  though  the  cold  set  in  at  a  different  time  from 
what  it  did  in  France.  There  were  40°  of  frost  at  Copenhagen  on  the 
12th  of  February,  or  the  temperature  was  — 7°'6  Fahr.  By  the  docu- 
ments which  M.  Renou  has  furnished  me  with  for  France,  I  discover  a 
minimum  of  — 9°'4  Fahr.  at  Perigueux,  and  of  —13°  Fahr.  at  Moulins! 
I  find  by  the  documents  supplied  me  by  Mr.  Glaisher,  that  he  also  con- 
siders the  winter  of  1870-1871  as  appertaining  to  the  class  of  winters 
memorable  for  their  severity. 

For  the  Seine  to  freeze  in  Paris  there  must  be  a  temperature  about 
+16°  Fahr.,  lasting  several  days.  We  have  seen  above  how  this  is 
brought  about.  Since  the  beginning  of  the  century  it  has  been  entire- 
ly frozen  over  eleven  times:  January,  1803 ;  December,  1812 ;  January, 
1820, 1821,  1823,  1829, 1830,  and  1838;  in  December,  1840;  in  Janu- 
ary, 1854 ;  and  in  January,  1865. 

M.  Eenou  has  noticed  that  the  severest  winters  seem  to  recur  about 
every  forty  years:  1709, 1749  (less  severe),  1789, 1830, 1870. 

The  following  are  the  lowest  temperatures  observed  in  France  since 
they  have  been  carefully  noted  by  the  thermometer.  They  are  in- 
scribed, like  the  previous  list  of  the  highest  temperatures,  in  geographic- 
al order  from  north  to  south.  I  have  taken  all  those  that  have  reach- 
ed 20°  of  frost,  and  only  those,  except  in  the  case  of  Paris,  where  there 
are  several  means  of  comparison. 


A  UTUMN—  WINTER. 


243 


Places. 

Latitude. 

Longi- 
tude. 

Alti- 
tude. 

Date. 

Minimum. 

Deg. 
50-22 

Deg. 
0-44 

Feet. 

78 

January     28  1776 

Deg. 

50*17 

0-26 

219 

December  30  1788 

lO'l 

Amiens  

49-53 

0'02 

118 

February   27  1776 

4*5 

Saint-Quentin 

49-50 

0*57 

341 

January     28  1776 

49-55 

1-34 

574 

December  31  1788 

7  '4 

49-39 

0-14 

324 

January     29  1776 

8  '5 

Rouen  

49-26 

1-15 

121 

December  30  1788 

7*2 

Clermont  (Oise)  

49-23 

0'05 

282 

"        26  1853 

4*0 

49-13 

1*37 

278 

4  '4 

Metz 

49-07 

3'50 

597 

"          31  1830 

4*9 

Montmorency  

49-00 

0-02 

600 

"                 1795 

4*0 

Chalons-sur-Marne  . 

48-57 

2*01 

269 

(December,       1788 

-  6-1 

Goersdorff 

48*57 

5  '26 

747 

(        "        26,  1853 
"        27  1853 

-  4*0 
7  '2 

Paris 

48  '50 

O'OO 

213 

"January     25,  1795 
13,  1709 
December  31,  1788 
February     6,  1665 

-10-3 
-  9'6 
-  7-2 
-  6*2 
3  *5 

48-48 

5*25 

213 

"          29,  1776 
December  30,  1783 
January     20,  1838 
"   "      17,  1830 
December        1788 

[-2-4 
-  2-2 

+  i-o 

—  6'7 

L'Aigle     . 

48-43 

2'00 

446 

"        30  1788 

7  '2 

48  '42 

3*51 

656 

(February     1,  1776 

-  8-7 

Strasbourg  

48*35  N. 

5*25 

472 

1       "           3,  1830 
JDecember  31,  1788 

-15-3 
—  15-3 

3§tampes      

48*26 

O'lO 

416 

(February     3,  1830 
December  31  1788 

—  10-1 
—   7'4 

Mayenne  

48-18 

2*57 

334 

"              1788 

—  4'0 

Troy  es  

48-18 

1-45 

360 

"        31,  1788 

—  9-4 

Saint-Die 

48'17 

4-37 

1125 

"        31  1788 

14*8 

Spinal 

48'10 

4'07 

1118 

February     3  1830 

—14-1 

Colmar  .  . 

48-05 

5*01 

639 

December  19  1788 

—  14'1 

Neuf  brissac  

48-00 

5'00 

649 

"        18,  1788 

—22-4 

Orleans 

47-54 

0*26 

403 

"        31  1788 

8'5 

47*49 

5'00 

751 

(January,          1784 

-  8-3 

Beaugency  

47-46 

0*46 

328 

{February     3,  1830 
December  31,  1788 

—  18-6 
—  8-5 

Tours 

47-24 

1'39 

180 

"        31  1788 

13'0 

Dijon  

47-19 

2'42 

807 

February     1,  1776 

—  4-0 

47*10 

2*06 

268 

December        1788 

—10*8 

47'05 

0'04 

511 

—  9-4 

Pontarlier 

46*54 

4*01 

2749 

(December  31,  1788 

-10-8 

46*40 

3'13 

846 

(        "        14,  1846 
j        "        31,  1788 

-24-3 
-11-2 

Poitiers 

46*35 

1*60 

387 

(January     16,  1838 
December        1788 

—  12'1 
—  4-0 

46*34 

I'OO 

744 

(December  31,  1788 

-  8-7 

Roanne  

46  '02 

1*44 

938 

\        "        22,  1870 
"        31,  1788 

-13*0 
—  6-1 

Limoges  

45-50 

1"05 

941 

"              1788 

-10-7 

Lyons  

45-46 

2-29 

967 

(        "        31,  1788 

-  7-4 

Grande-Chartreuse  
Grenoble 

45-48 
45-11 

3-23 
3'24 

6660 
698 

(January     16,  1838 
December  30,  1788 
February,         1776 

—  4'0 
-15-3 
6-9 

45'H 

1'36 

321 

December,       1870 

-  9-4 

Aurillac  

44-56 

0-06 

2040 

December  27,  1829 

—10-5 

The  greatest  cold  yet  experienced  has  been  -24°  in  France;  —5°  in 


244  THE  ATMOSPHERE. 

England ;  —12°  in  Holland  and  Belgium ;  —67°  in  Denmark,  Sweden, 
and  Norway ;  -46°  in  Eussia ;  -32°  in  Germany ;  zero  in  Italy  ;  -10° 
in  Spain  and  Portugal.  As  to  other  countries,  not  European,  more  ob- 
servations must  be  taken  before  one  can  speak  with  any  degree  of  cer- 
tainty upon  the  point.  It  is,  nevertheless,  certain  that  at  Fort  Reliance, 
in  British  North  America,  there  have  been  —70°  of  cold,  and  at  Semi- 
palatinsk  —76°.  Quicksilver  freezes  at  —39°.  There  are  inhabited 
points  of  the  globe  where  it  remains  congealed  for  several  months  of  the 
year — on  Melville  Island,  for  instance.  Captain  Parry,  moreover,  as- 
serts that  a  person  sufficiently  wrapped  up  may  safely  expose  himself 
to  the  open  air  in  —50°,  or  82°  below  freezing-point  of  water— that  is, 
if  there  is  no  wind.  In  this  latter  event  the  skin  is  rapidly  affected. 
Frozen  mercury  looks  like  lead,  but  it  is  not  so  hard,  is  more  fragile, 
and  less  coherent.  If  touched  it  burns  like  hot  iron.  Small  statu- 
ettes can  be  made  with  it  which  melt  when  the  temperature  is  higher 
than  -39°. 

Such  are  the  greatest  frosts  that  have  been  experienced.  If  they  are 
compared  with  the  extremes  of  heat  noticed  in  the  previous  chapter 
(165°  upon  the  surface  of  the  soil  of  Africa),  it  will  be  seen  that  the  ex- 
tremes of  temperature  upon  the  globe  may  attain  a  scale  of  nearly  240 
degrees. 


CLIMATE.  245 


CHAPTER  VI. 

CLIMATE  :  DISTRIBUTION"  OF  TEMPERATURE  OVER  THE  GLOBE  —  ISO- 
THERMAL LINES  —  THE  EQUATOR — THE  TROPICS — THE  TEMPERATE 
REGIONS — THE  POLES — THE  CLIMATE  OF  FRANCE. 

IF  two  lines  parallel  to  the  equator  be  traced  upon  the  globe,  at  the 
distance  of  23°  28'  in  each  hemisphere,  they  will  mark  two  circles  be- 
tween which  the  sun  is  seen  to  pass  across  the  zenith  at  certain  epochs 
of  the  year;  these  are  the  tropics.  That  of  the  northern  hemisphere  is 
known  as  the  Tropic  of  Cancer,  because,  during  the  summer  solstice,  the 
sun  passes  at  its  zenith  and  is  in  the  zodiacal  sign  of  Cancer.  That  of 
the  southern  hemisphere  is  known  as  the  Tropic  of  Capricorn,  because 
the  sun  passes  at  its  zenith  during  the  winter  solstice  in  the  zodiacal 
sign  of  Capricorn.  The  zone  included  between  these  two  circles  is  the 
hottest  part  of  the  earth,  inasmuch  as  it  comprises  the  places  over  which 
the  sun  rises  to  its  greatest  altitudes ;  it  is  termed  the  torrid  or  inter- 
tropical  zone. 

If  two  other  circles,  distant  23°  28'  from  the  pole,  or  at  66°  32' 
from  the  equator,  be  drawn  upon  this  same  terrestrial  globe,  they  will 
mark  the  points  below  which  the  sun  may  remain  for  several  days  to- 
gether, and  above  which  it  remains  at  its  least  altitudes ;  these  are  the 
polar  circles.  During  one  half  of  the  year  the  sun  rises  spirally  above 
them  to  the  height  of  23°  28',  and  during  the  other  half  descends  below 
them  to  the  same  amount.  Between  these  two  zones  is  the  temperate 
zone,  in  respect  to  which  the  sun  rises  and  sets  each  day,  without  ever 
reaching  so  high  as  the  zenith,  attaining  an  increasing  elevation,  and 
giving  a  greater  length  of  day,  so  far  as  our  hemisphere  is  concerned, 
from  the  solstice  of  December  to  the  solstice  of  June,  corresponding  with 
which  there  is  an  inverse  rate  of  progress  in  the  other  hemisphere. 

The  two  glacial  zones  form  82  thousandths  of  the  surface  of  the 
earth ;  the  two  temperate  zones  represent  520  thousandths ;  and,  finally, 
the  torrid  zone,  composed  of  the  two  regions  comprised  between  the 
tropics  and  the  equator,  is  398  thousandths  of  the  whole  surface  of  our 
planet. 

The  length  of  the  longest  and  the  shortest  days,  in  the  different  lati- 


246 


THE  ATMOSPHERE. 


tudes  of  our  hemisphere,  from  the  equator  to  the  polar  circles,  gives 
the  following  scale : 


Latitudes. 

Names  of  Places. 

Longest  Day. 

Shortest  Day. 

Deg. 

(  Quito")                                                     

hrs.  min. 
12     0 

hrs.  min. 
12    0 

5 

12  17 

11  43 

12  35 

11  25 

(St  Louis)                               

12  53 

11     7 

20 

13  13 

10  47 

25 

(Canton)                          

13  34 

10  26 

Of) 

(Cairo)                                     

13  56 

10    4 

35 

(Algiers)                         

14  22 

9  38 

(Madrid  Naples) 

14  51 

9    9 

AK 

15  26 

8  34 

KO 

(Dieppe  Frankfort)       

16     9 

7  51 

55 

(  Edinburgh  Copenhagen)  

17    7 

6  53 

18  30 

5  30 

(Archangel) 

21     9 

2  51 

66-32 

(Polar  Circle)  

24    0 

0    0 

It  is  of  course  the  same  in  the  southern  hemisphere.  Beyond  the  polar 
circles  the  length  of  the  day  varies  from  0  to  24  hours  in  that  part  of 
the  year  during  which  the  sun  rises  or  sets.  The  number  of  days  dur- 
ing which  the  sun  is  constantly  above  or  constantly  below  the  horizon 
in  different  latitudes,  from  66°  32'  to  90°,  is  given  in  the  following  table, 
the  phenomena  being  just  the  reverse  for  the  two  glacial  zones: 


Latitudes. 

The  Sun  does  not  set 
in  the  Northern  Hemi- 
sphere nor  rise  in  the 
Southern  during 

The  Sun  does  not  rise 
in  the  Southern  Hemi- 
sphere nor  set  in  the 
Northern  during 

Deg. 
66-32 
70 
75 
80 
85 
90 

Days. 
1 
65 
103 
134 
161 
186 

Days. 
1 
60 
97 
127 
153 
179 

In  this  theory  of  the  climates  we  suppose  the  sun  to  be  reduced  to  a 
point  at  its  centre;  we  have,  moreover,  neglected  to  take  into  account 
the  phenomena  of  starlight  produced  by  the  refraction  of  light.  As  the 
diameter  of  the  sun  is  about  32',  the  latitude  at  which  it  would  disap- 
pear altogether  must  be  placed  at  16'  farther  back.  The  refraction,  too, 
raising  it  by  33'  at  the  horizon,  the  absolute  polar  circles  must  be  also 
placed  that  distance  farther  back.  Lastly,  night  is  not  complete  until 
the  sun  has  descended  to  about  18°  below  the  horizon ;  and  this  cir- 
cumstance must  also  be  taken  into  account,  whence  it  results  that  near 
the  poles  day  is  hardly  ever  extinct,  and  night,  in  the  absolute  sense  of 
the  term,  almost  unknown. 


CLIMATE. 


247 


The  seasons  are  exactly  opposite  in  the  two  hemispheres,  as  we  have 
already  stated ;  they  are  indeed  neither  more  nor  less  than  the  intervals 
of  time  which  the  earth  takes  to  traverse  the  four  parts  of  its  orbit  com- 
prised between  the  equinoxes  and  the  solstices.  In  consequence  of  the 
eccentricity  of  the  earth's  orbit,  and  by  virtue  of  the  law  of  superficies, 
the  lengths  of  the  seasons  differ ;  they  are  represented  by  the  following 
figures,  which  show  that  the  sun  is,  in  the  course  of  each  year,  about 
eight  days  longer  in  the  northern  hemisphere  than  in  the  southern 
hemisphere : 


Days. 

Hrs. 

Min. 

Autumn  (September  22  to  December  21)  

89 

18 

35 

Winter  (December  21  to  March  21)  

89 

0 

2 

Sojourn  of  the  sun  in  the  southern  hemisphere... 

178 

18 

37 

Spring  (March  21  to  June  21) 

92 

20 

59 

Summer  (June  21  to  September  22)  

93 

14 

13 

Sojourn  of  the  sun  in  the  northern  hemisphere... 

186 

11 

12 

The  sun  being,  in  fact,  the  source  of  heat  for  the  surface  of  the  earth, 
it  follows  that  the  hottest  countries  are  those  over  which  it  remains 
the  longest,  and  upon  which  it  darts  its  rays  the  most  vertically,  that 
is,  the  regions  situated  along  the  equator,  and  upon  each  side  of  it,  as 
far  as  the  tropics.  Thus,  these  warm  regions  are  known  by  the  generic 
appellation  of  "  the  torrid  zones."  In  proportion  as  one  recedes  from 
the  equator  and  approaches  the  poles,  it  is  seen  that  the  sun  attains  a 
lesser  elevation,  and  that  for  six  months  the  nights  are  longer  than  the 
days;  these  are  the  temperate  regions,  where  the  seasons  lend  a  far 
greater  variety  to  the  products  of  nature,  but  where  the  mean  of  the 
annual  temperature  gradually  diminishes  according  to  the  diminution 
in  the  apparent  height  of  the  sun  at  noon.  Lastly,  when  we  pass  be- 
yond 66°  of  latitude,  the  glacial  polar  region  is  reached  over  which  the 
sun,  even  on  the  finest  days,  scarcely  rises  sufficiently  high  to  melt  the 
eternal  ice  subsisting  in  these  regions. 

It  is  superfluous  for  me  to  mention  that  the  south  pole  is  cold  like 
the  north  pole,  noth withstanding  the  idea  attaching  to  this  direction. 
Some  few  poets  still  talk  of  traveling 

"Du  pole  brulant  jusqu'au  pole  nord;" 

but  such  metaphors  are  no  longer  admissible.  The  equator  is  to  the 
south  of  our  position,  and  the  winds  blowing  from  there  toward  us  are 
hot.  The  equator  is  to  the  north  of  the  other  hemisphere,  and  the 


248  THE  ATMOSPHERE. 

winds  which  reach  it  from  the  equator  are  also  hot,  though  they  come 
from  the  north.  In  respect  to  meteorological  direction,  as  in  regard  to 
the  seasons,  the  inhabitants  of  Australia,  the  Cape  of  Good  Hope,  Cape 
Horn,  Buenos  Ayres,  and  Santiago  feel  and  speak  just  contrary  to  what 
we  do. 

Latitude— that  is,  the  angle  at  which  the  solar  rays  reach  the  surface 
of  the  ground— being  the  great  cause  of  the  succession  of  climates  from 
the  equator  to  the  poles,  the  diminution  would  be  progressive  and  reg- 
ular if  the  earth  was  a  globe,  perfectly  regular  in  shape,  instead  of  being 
divided  into  earth  and  water,  and  broken  by  mountains,  table-lands, 
and  valleys.  The  quantity  of  heat,  estimated  at  1000,  for  instance,  at 
the  equator,  would  follow  a  constantly  descending  scale,  marking  923 
at  each  of  the  tropics,  720  at  the  latitude  of  Paris,  and  500  at  the  polar 
circle.  But  the  earth  is  not  a  smooth  and  undisturbed  sphere,  and  rev- 
olutions more  or  less  harmonious  are  constantly  occurring. 

We  shall  see  in  this  work  that  the  atmosphere  is  in  a  perpetual  state 
of  circulation,  and  that  there  are  general  winds  which  periodically  trav-. 
erse  different  countries  of  the  globe.  These  regular  currents  modify 
the  normal  distribution  of  climates.  Thus  the  trade-winds,  which  es- 
tablish a  double  current  between  the  equator  and  the  poles,  temper  at 
once  the  cold  of  the  high  latitudes  over  which  they  pass  and  the  heat 
of  the  tropical  regions,  heating  the  former  and  cooling  the  latter. 

A  second  cause  is  added  to  this  by  which  the  temperature  along  the 
same  circles  of  latitude  is  varied.  The  globe  is  divided  into  oceans  and 
continents.  Water  has  a  greater  capacity  for  heat  than  land,  whence 
it  results  that  the  sea  is  cooler  than  the  land  in  summer,  and  warmer  in 
winter.  The  winds  which  blow  from  the  sea  prevent  the  coast-line  from 
being  as  cold  as  the  country  farther  inland.  As  the  south-west  wind  is 
that  which  blows  oftenest,  the  western  coasts  of  Spain,  France,  Scotland, 
and  Norway  are  warmer  than  the  inland  country  in  the  same  latitudes. 
The  great  marine  current  known  as  the  Gulf  Stream  adds  still  further 
to  this  modifying  cause. 

,Water  becomes  less  readily  heated  upon  the  surface  than  earthy  mat- 
ter, because  the  latter  has  a  specific  heat  much  below  that  of  water;  so 
that  the  quantity  of  solar  heat  required  to  raise  its  temperature  by  10°, 
for  instance,  is  much  less  than  that  which  would  raise  the  temperature 
of  a  liquid  surface  the  same  number  of  degrees. 

It  must,  furthermore,  be  mentioned  that  the  solar  rays  which  become 
absorbed  in  a  very  thin  layer  of  earth  penetrate,  at  least  in  part,  to  a 


CLIMATE.  249 

very  considerable  depth  in  the  water ;  that  at  sea,  especially,  they  do 
not  become  totally  extinguished  until  they  nave  reached  a  depth  of 
more  than  three  hundred  yards ;  so  that  the  heat  arising  from  absorp- 
tion, instead  of  being  concentrated  upon  the  surface,  is  spread  over  a 
great  mass  of  water,  and  must  be  the  less  in  proportion  as  this  mass  is 
larger. 

Evaporation,  which  is,  as  we  have  seen,  a  very  great  cause  of  cold,  is 
the  greater  according  as  this  phenomenon  occurs  upon  a  larger  scale. 
And,  where  the  liquid  mass  continually  furnishes  the  means  of  evapo- 
ration, there  exists  a  cause  of  cold  which  does  not  exist  at  all,  or  in  a 
much  less  degree,  upon  dry  land.  From  these  three  causes  (specific 
heat,  diathermacy,  and  evaporation)  it  follows  that  water,  and  the  at- 
mosphere which  is  in  contact  with  it,  must  be  less  heated  than  conti- 
nental regions  situated  in  the  same  latitude.  In  winter,  on  the  other 
hand,  it  is  warmer,  and  that  for  a  reason  which  it  is  easy  to  compre- 
hend. 

I  have  already  stated  that  the  superficial  moleculae,  rendered  cold  by 
their  radiation  toward  the  cold  regions  of  space,  are  precipitated  toward 
the  bottom  by  reason  of  the  excess  of  their  specific  weight;  consequent- 
ly the  surface  of  the  sea  must  preserve  a  higher  temperature  than  that 
of  the  surface  of  continents,  since  in  this  case  the  superficial  moleculae 
that  have  become  cold  do  not  plunge  into  the  ground. 

These  consequences,  deduced  from  a  minute  examination  of  the  ac- 
tion of  the  solar  rays  upon  a  liquid  and  a  continental  surface,  are  con- 
firmed by  observation. 

Thus,  at  Bordeaux,  the  mean  winter  temperature  is  42°'8 ;  whereas, 
in  the  same  latitude,  the  temperature  of  the  Atlantic  never  falls  below 
51°-3. 

In  latitude  50°  the  ocean  has  never  been  known  to  be  less  than  480>2. 

The  mass  of  observations  collected  show  that,  in  the  northern  hemi- 
sphere and  in  the  temperate  zone,  the  mean  temperature  of  an  island 
situated  in  the  midst  of  the  Atlantic  would  be  higher  than  the  mean 
temperature  of  a  spot  similarly  situated  upon  the  main-land — that  the 
winter  would  be  warmer  and  the  summer  cooler.  This  has  been  espe- 
cially remarked  in  the  Island  of  Madeira. 

The  sea  serves  to  equalize  the  temperatures.  Hence  there  is  an  im- 
portant difference  between  the  climate  of  islands  or  coast-lands  peculiar 
to  all  continents  that  abound  in  gulfs  and  peninsulas,  and  the  climate 
of  the  interior  of  a  great  and  compact  mass  of  dry  land.  In  the  interi- 


250  THE  ATMOSPHERE. 

or  of  Asia,  at  Tobolsk,  Barnaul-upon-Obi,  and  Irkoutsk,  the  summer  is 
the  same  as  at  Berlin,  Miinster,  and  Cherbourg ;  but  these  summers  are 
followed  by  winters  when  the  temperature  is  as  low  as  —  0°-4  or  —4°. 
During  the  summer  months  the  thermometer  will  remain  for  weeks  to- 
gether at  86°  or  88°.  These  continental  climates  have  been  very  appro- 
priately termed  excessive  by  Buffon,  and  the  inhabitants  of  countries  in 
which  they  prevail  seem  to  be  condemned,  like  the  spirits  alluded  to  by 
Dante, 

"A  sofferir  tormenti  e  caldi  e  giek. 

The  climate  of  Ireland,  of  Jersey  and  Guernsey,  of  the  peninsula  of 
Brittany,  of  the  coasts  of  Normandy,  and  the  south  of  England,  coun- 
tries in  which  the  winters  are  mild  and  the  summers  cool,  contrasts 
very  strikingly  with  the  continental  climate  of  the  interior  of  Eastern 
Europe.  In  the  north-east  of  Ireland  (54°'56),  in  the  same  latitude  as 
Konigsberg,  the  myrtle  grows  in  the  open  ground  just  as  it  does  in 
Portugal.  The  temperature  of  the  month  of  August  in  Hungary  is 
69°'8 ;  in  Dublin  (upon  the  same  isothermal  line  of  49°)  it  is  61  de- 
grees at  most.  The  mean  temperature  of  winter  descends  to  360>3  at 
Buda.  In  Dublin,  where  the  annual  temperature  is  only  49°,  that  of 
the  winter  is,  nevertheless,  7°7  above  the  freezing-point,  or  nearly  four 
degrees  higher  than  at  Milan,  Pavia,  Padua,  and  all  Lombardy,  where 
the  mean  heat  of  the  year  reaches  55°.  In  the  Orkney  Islands,  at 
Stromness,  a  little  to  the  south  of  Stockholm  (there  is  not  one  degree 
difference  in  latitude),  the  mean  winter  temperature  is  7°,  or  higher 
than  that  of  London  or  Paris.  Stranger  still,  the  inland  waters  of  the 
Faroe  Islands  never  freeze,  situated  in  62°  of  north  latitude,  beneath 
the  mild  influences  of  the  west  wind  and  the  sea.  Upon  the  coast  of 
Devonshire,  one  part  of  which  has  been  termed  the  Montpellier  of  the 
North,  because  of  the  mildness  of  its  climate,  the  Agave  Mexicana  has 
been  known  to  flower  when  planted  in  the  open  air,  and  orange-trees 
trained  upon  a  wall  to  bear  fruit,  though  only  scantily  protected  by  a 
thin  matting.  There,  as  at  Penzance,  Gosport,  Cherbourg,  and  the 
coast  of  Normandy,  the  mean  temperature  of  winter  is  42°,  being  but 
18°-5  below  that  of  Montpellier  and  Florence. 

The  mean  annual  temperature  of  London,  as  deduced  by  Glaisher 
from  one  hundred  years'  observations  (1771-1870),  is  48°'5.  The  mean 
summer  temperature  is  60° '2,  and  that  of  winter  38°.  The  winter, 
therefore,  is  warmer  at  London  than  at  Paris,  and  the  summer  and  the 
year  cooler.  Although  Cherbourg  is  one  degree  of  latitude  north  of 


CLIMATE.  251 

Paris,  its  mean  temperature  is,  notwithstanding,  higher,  being  52°-3, 
while  that  of  Paris  is  only  51° '3.  The  difference  between  the  winter 
climates  of  the  two  towns  is  much  greater,  since  the  winter  mean  is 
43°-7  at  Cherbourg,  and  37°'8  at  Paris.  Thus  fig-trees,  laurels,  and 
myrtles,  which  would  perish  in  the  neighborhood  of  Paris,  are  found  to 
flourish  in  the  former  place.  The  enormous  fig-trees  which  grow  at 
Eoscoff,  in  Brittany,  are  almost  equal  to  those  of  Smyrna. 


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Fig.  54. — Comparative  temperatures  of  the  European  capitals  of  Rome,  London,  Paris,  Vienna, 
St.  Petersburg. 

These  comparisons  are  sufficient  evidence  as  to  how  the  same  mean 
annual  temperature  may  be  distributed  in  many  different  proportions 
over  the  various  seasons,  and  how  great  an  influence  these  diverse 
modes  of  distribution  of  heat  may  exercise  in  the  course  of  the  year 
upon  vegetation,  agriculture,  the  ripening  of  fruit,  and  the  comfort  of 
man. 

The  same  relations  of  climate,  which  are  remarked  as  existing  be- 
tween the  peninsula  of  Brittany  and  the  rest  of  France,  the  mass  of 
which  is  more  compact,  and  where  the  summers  are  hotter,  and  the 
winters  colder,  are  reproduced  to  a  certain  extent  as  between  Europe 
and  the  continent  of  Asia.  Europe  owes  the  mildness  of  its  climate  to 
its  abundantly  indented  configuration,  to  the  ocean  which  washes  its 
western  shores,  to  the  sea  which  separates  it  from  the  polar  regions, 
and,  above  all,  to  the  existence  and  to  the  geographical  situation  of  the 
African  continent,  the  intertropical  regions  of  which  radiate  excessively 


252  THE  ATMOSPHERE. 

and  cause  the  ascent  of  an  immense  current  of  hot  air,  whereas  the  re- 
gions situated  to  the  south  of  Asia  are,  for  the  most  part,  oceanic. 

Europe  would  become  colder  if  Africa  were  submerged— if  the  fabled 
Atlantides,  emerging  from  the  ocean,  were  to  unite  Europe  to  America 
— if  the  warm  waters  of  the  Gulf  Stream  did  not  flow  into  the  northern 
seas,  or  if  a  new  land,  upheaved  by  volcanic  agency,  were  to  become  in- 
serted between  the  Scandinavian  Peninsula  and  Spitzbergen.  In  pro- 
portion as  we  advance  from  west  to  east,  along  the  same  latitude,  in 
France,  Germany,  Poland,  Eussia,  and  as  far  as  the  Ural  Mountains,  we 
find  that  the  mean  annual  temperatures  follow  a  uniformly  descending 
scale.  But  as  we  penetrate  inland,  the  form  of  the  continent  becomes 
more  and  more  compact,  its  breadth  increases,  the  influence  of  the 
sea  diminishes,  and  that  of  the  west  wind  becomes  less  perceptible. 
Therein  lies  the  chief  cause  of  the  progressive  decline  in  the  tem- 
perature. 

The  mean  temperature  of  the  equator  is  810<5.  Owing  to  the  causes 
which  I  have  specified,  and  to  the  absence  of  vegetation,  that  of  inland 
Africa  is  86  degrees  with  the  thermometer  placed  in  the  shade  and  pro- 
tected from  hot  winds ;  but  there  are  points  at  which  the  action  of  the 
burning  breeze,  and  the  absence  of  clouds,  combine  in  producing  an  in- 
tolerable degree  of  heat.  Thus,  in  the  interior  of  Abyssinia,  and  in  the 
neighborhood  of  the  Red  Sea,  it  is  by  no  means  rare  to  meet  with  a 
summer  temperature  of  118  to  122  degrees  in  the  shade.  That  of  the 
soil  is  higher  still.  In  the  afternoon  the  valleys  of  Abyssinia  are  regu- 
lar furnaces;  M.  d'Abbadie  having  observed  the  temperature  of  the  soil 
at  160  degrees  nearly,  while  Colonels  Ferret  and  Galinier  met  with  a 
temperature  of  167  degrees.  The  air  is  stagnant  in  the  midst  of  all  this 
heat,  and  there  is  no  refreshing  breeze.  The  air  in  the  depths  of  these 
ravines  is  often  mephitic,  and  to  repose  therein  after  or  before  the  rainy 
season  is  fatal.  It  is  necessary  at  that  period  to  travel  by  night,  as 
plains  have*  to  be  crossed  which  afford  no  place  of  shelter. 

"Sometimes  in  crossing  these  deserts,"  says  M.  d'Abbadie,  "one  is 
assailed  by  the  karif,  a  sort  of  aerial  hurricane,  a  phantom  of  burning 
dust  which  appears  upon  the  horizon,  and  seems  to  grow  in  size  as  it 
approaches.  The  wind  which  wafts  it  blows  like  a  hurricane;  men  and 
animals  are  obliged  to  turn  their  backs,  and  are  enveloped  in  a  dry  and 
black  cloud,  which  covers  them  as  with  a  hideous  mantle.  Fortunately 
this  storm  of  fire  lasts  only  a  few  minutes,  and  after  it  has  passed  the 
intense  heat  which  is  peculiar  to  these  regions  is  felt  as  a  relief. 


CLIMATE.  253 

"At  other  times  one  is  overtaken  by  the  simoom  (the  poison),  a  wind 
of  flame  which  begins  to  blow  without  any  premonitory  sign.  The 
camel  is  then  seen  to  lay  his  head  upon  the  ground,  seeking  coolness 
from  it,  though  it  is  itself  like  a  furnace.  The  hardiest  of  the  natives 
are  struck  down  in  despair.  The  prostration  is  so  great,  that  I  was 
myself  unable  to  lift  a  small  thermometer  placed  within  my  reach,  in 
order  to  ascertain,  at  all  events,  the  temperature  of  this  remarkable 
wind.  Its  duration  was  five  minutes:  it  causes  death  when  it  continues 
for  a  quarter  of  an  hour. 

"  If  one  happens  to  meet  with  a  small  stream  in  these  regions,  it  soon 
disappears,  absorbed  by  the  sand.  These  miniature  oases,  composed  of 
a  few  trees  and  some  grass,  are  very  rare. 

"  These  same  valleys  are  the  theatre  of  a  very  extraordinary  phenom- 
enon ;  a  sudden  irruption  of  water,  which  at  certain  periods  of  the  year 
causes  inundations,  to  which  those  occurring  in  European  countries  are 
trifling.  And,  strange  to  relate,  they  take  place  during  the  summer. 

"  One  may  be  traveling  with  a  full  sense  of  security,  when  a  native, 
hearing  a  strange  and  distant  noise,  commences  to  shout  at  the  top  of 
his  voice,  '  The  torrent !'  and  climbs  as  fast  as  possible  to  the  nearest 
elevated  point.  In  a  few  seconds  the  hollow  of  the  valley  is  hidden  by 
a  deep  body  of  water,  which  carries  with  it  trees,  rocks,  and  even  wild 
animals.  These  torrents,  formed  in  an  instant,  disappear  in  the  course 
of  the  same  day,  and  leave  no  trace  of  their  passage,  save  debris  and 
muddy  deposits. 

"  How  is  this  strange  phenomenon  to  be  explained  ?  The  barrenness 
of  the  mountains  accounts  for  these  sudden  down-pours.  From  the 
hollow  of  the  ravine  in  which  the  traveler  is  journeying,  he  is  unable 
to  see  the  narrow  clouds  which  suddenly"  dissolve  into  water,  with  an 
abundance  unknown,  save  in  tropical  regions.  There  is  very  little  soil, 
and  still  fewer  roots  of  trees,  to  absorb  this  sudden  rain,  which  conse- 
quently runs  off  at  once,  leaping  from  rock  to  rock,  as  down  a  roof, 
flowing  from  each  minor  valley  into  the  principal  ravine,  and  there 
forming  a  stream  which,  though  short-lived,  is  of  mighty  dimensions." 

M.  d'Abbadie  further  relates  how  he  was  just  too  late  on  one  occasion 
to  witness,  in  all  its  grandeur,  one  of  these  sudden  inundations.  He 
found  a  native,  who  was  regarding  the  wet  ground  with  the  air  of  one 
who  had  been  stunned.  "Peace  be  with  you,"  said  M.  d'Abbadie; 
"  what  news?  Where  are  your  arms?  Surely  you  can  not  be  without 
your  lance  and  shield?"  "Peace  be  with  you,"  replied  the  African, 


254  THE  ATMOSPHERE. 

"  the  torrent  has  carried  off  my  lance,  my  shield,  my  camel,  and  all  my 
fortune,  my  wife  and  my  children." 

It  will  thus  be  seen  that  various  causes  influence  the  climates  of  dif- 
ferent countries  of  the  globe ;  and  it  would  involve  great  errors  were 
we  to  take  into  account  the  distance  from  the  equator  only  in  calcula- 
ting the  decrease  of  the  temperature  toward  the  poles.  We  have  seen 
that  the  average  temperature  of  the  equator  is  81°'5 ;  the  mean  temper- 
ature of  Paris  is  51°'3 ;  that  of  regions  within  the  polar  circle  about  5°. 

To  establish  a  correct  table  of  the  distribution  of  temperature  over 
the  surface  of  the  earth,  Humboldt  marked  upon  a  map  all  the  points 
at  which  reliable  thermometrical  observations  had  been  taken,  noted 
the  degrees  recorded,  and  then  traced  lines  passing  respectively  through 
all  the  places  where  the  mean  temperature  was  the  same.  These  he 
termed  isothermal  lines  (from  to-oe,  equal,  and  Oepfiog,  heat).  During 
the  fifty  years  since,  observations  have  been  multiplied  and  the  maps 
made  more  perfect. 

We  see  in  diagrams  of  isothermal  lines,  or  lines  of  equal  temperature 
running  along  the  western  shores  of  Europe,  that  the  line  of  50°,  for  in- 
stance, touches  the  fortieth  degree  of  latitude  south-west  of  New  York, 
and  reaches  as  far  as  55°  near  England ;  so  that  Dublin  and  London 
have  nearly  the  same  mean  temperature  as  New  York,  although  they 
are  situated  much  farther  north ;  the  same  temperature  then  falls  again 
toward  the  south,  passing  to  Vienna,  Astrakhan,  and  Pekin,  and  de- 
scending even  below  the  fortieth  parallel  of  latitude.  The  greatest  heat 
line,  called  the  thermic  equator,  is  nearly  entirely  to  the  north  of  the 
equator,  and  its  temperature  varies,  according  to  situation,  from  81°  to 
86°.  Within  the  polar  regions  the  mean  temperature  of  different  places 
decreases  to  as  much  as  1°,  which  has,  as  yet,  scarcely  been  traced,  in 
consequence  of  the  difficulty  of  traveling  in  these  inhospitable  regions. 

Humboldt  has  pointed  out  that,  notwithstanding  these  great  differ- 
ences, the  mean  temperature  decreases  almost  uniformly  at  the  rate  of 
nearly  a  degree  of  the  thermometer  to  each  degree  of  latitude.  But  as, 
on  the  other  hand,  the  heat  diminishes  by  1°  for  an  increase  of  height 
of  about  three  hundred  feet,  it  follows  that  an  elevation  of  about  one 
hundred  yards  produces  the  same  effect  upon  the  temperature  of  the 
year  as  an  approach  of  1°  of  latitude  toward  the  north.  Thus,  the  mean 
annual  temperature  of  the  monastery  of  Mount  St.  Bernard,  situated  at 
a  height  of  8173  feet,  in  latitude  45°  50',  is  the  same  as  that  of  low 
ground  in  75°  50'  latitude.  By  studying  the  distribution  of  heat  over 


CLIMATE.  255 

the  surface  of  the  globe,  and  by  tracing  a  system  of  isothermal  lines, 
Humboldt  demonstrated  the  causes  which  raise  the  temperature  of  a 
particular  spot,  and  those  which  lower  it.  The  augmenting  causes  are 
as  follows : 

The  proximity  of  the  ocean  on  the  west  in  the  temperate  zone.  The 
configuration  peculiar  to  continents  which  are  cut  up  into  numerous 
peninsulas.  The  Mediterranean,  and  the  gulfs  penetrating  far  inland. 
The  direction,  that  is  to  say,  the  position  of  a  country  in  respect  to  a 
sea  free  from  ice,  which  extends  beyond  the  polar  circle,  or  in  regard 
to  a  continent  of  considerable  extent,  situated  upon  the  same  meridian, 
at  the  equator,  or  at  least  in  the  interior  of  the  tropical  zone.  The 
south-westerly  direction  of  the  prevailing  winds  in  the  case  of  the  west- 
ern fringe  of  a  continent  situated  in  the  temperate  zone,  the  chains  of 
mountains  acting  as  a  rampart  and  a  protection  against  the  winds  which 
blow  from  colder  countries.  The  scarcity  of  pieces  of  water,  the  sur- 
face of  which  is  covered  with  ice  during  the  spring,  and  up  to  the  be- 
ginning of  summer.  The  absence  of  forests  on  a  dry  and  sandy  soil,  the 
constant  serenity  of  the  sky  during  the  summer  months,  and,  lastly,  the 
near  neighborhood  of  a  maritime  current,  whose  waters  are  warmer  than 
those  of  the  surrounding  ocean. 

The  decreasing  causes  are :  the  height  above  the  level  of  the  sea  of  a 
region  which  does  not  possess  extensive  table-land.  The  distance  of 
the  sea  to  the  west  and  the  south  in  our  hemisphere.  The  compact 
shape  of  a  continent,  upon  the  coasts  of  which  there  are  no  bays ;  a 
great  extent  of  land  toward  the  pole,  and  toward  the  regions  of  eternal 
frost,  except  in  the  case  of  there  being  between  the  land  and  this  region 
a  sea  that  is  free  of  ice  during  the  winter ;  a  geographical  position  such 
that  the  tropical  regions  in  the  same  longitude  are  covered  by  the  sea : 
in  other  words,  the  absence  of  any  tropical  land  upon  the  meridian  of 
the  country  whose  climate  is  being  studied ;  a  chain  of  mountains  which 
by  its  shape  or  direction  prevents  the  access  of  warm  winds,  or  indeed 
the  presence  of  isolated  peaks,  because  in  both  these  cases  currents  of 
cold  air  make  their  way  down  the  slopes ;  forests  of  great  extent,  for 
these  prevent  the  solar  rays  from  acting  upon  the  soil ;  the  leaves  cause 
the  evaporation  of  large  quantities  of  water,  by  reason  of  their  organic 
activity,  and  increase  the  superficies  liable  to  be  rendered  cold  by  radia- 
tion. The  forests  act,  therefore,  in  three  ways :  by  their  shade,  by  their 
evaporation,  and  by  their  radiation.  The  numerous  pieces  of  water 
which,  in  the  north,  are  regular  receptacles  of  ice  up  to  the  middle  of 


256  THE  ATMOSPHERE. 

summer.  A  cloudy  sky  in  summer,  because"  it  intercepts  a  portion  of 
the  sun's  rays ;  a  very  clear  sky  in  winter,  because  it  facilitates  the  ra- 
diation of  the  heat. 

To  the  general  conditions  of  climates  must  be  added  the  influence 
which  local  circumstances  may  have  upon  the  state  of  the  temperature. 
It  is  far  more  difficult  than  is  generally  supposed  to  ascertain  exactly  the 
temperature  of  a  given  spot  upon  the  surface  of  the  globe,  and  especially 
of  an  inhabited  spot ;  for  ten  thermometers,  identically  the  same,  and 
carefully  compared,  will  not  mark  the  same  point  at  the  same  moment 
in  ten  different  streets  of  the  same  town.  The  principal  remark  to  be 
made  in  reference  to  this  is,  that  in  consequence  of  the  radiation  of 
dwelling-houses,  and  on  account  of  the  obstacles  which  an  agglomera- 
tion of  buildings  puts  in  the  way  of  free  circulation  of  air,  the  tempera- 
ture of  large  towns  is  always  less  marked  and  higher  than  that  of  the 
country  around  them.  Howard  showed  that  the  mean  temperature  of 
London  exceeds  by  2°  that  of  the  surrounding  district.*  The  ther- 
mometers of  the  Paris  Observatory  are  never  so  high  as  those  in  the 
heart  of  the  city,  but  are  higher  than  those  placed  in  the  open  air  in  the 
field  adjoining.  Every  one  has  noticed  that  it  is  cooler  in  summer  and 
warmer  in  winter  in  the  narrow  streets  of  old  Paris  than  upon  the  mod- 
ern squares  and  boulevards.  There  is  frequently  a  difference  of  sev- 
eral degrees.  Even  in  the  open  country,  at  the  same  altitude  and  in  the 
same  frontage,  the  temperature  differs  according  to  the  distance  from 
woods.  These  latter  act  upon  the  temperature  of  the  air,  which  is  low- 
er in  than  it  is  outside  them.  The  mean  maxima  outside  of  woods  are 
higher  than  inside.  The  mean  temperature  of  summer  is  also  higher  in 
the  former  case  than  in  the  latter.  These  facts  are  clearly  shown,  ac- 
cording to  MM.  Becquerel,  by  the  results  of  more  than  fourteen  thou- 
sand observations  made  during  the  last  few  years. 

The  hours  of  maxima  and  minima  are  not  the  same  inside  the  trees 
(even  when  they  stand  alone)  as  in  the  open  air.  They  vary  according 
to  the  kind  and  the  diameter  of  the  tree.  The  variations  of  temperature 
among  the  leaves  are  about  the  same  as  those  in  the  surrounding  air; 
in  the  young  branches  they  occur  more  slowly,  and  so  on  to  the  trunk, 
where  they  are  very  gradual.  I  am  excluding  from  the  question  the 

[*  In  a  paper  published  in  the  Philosophical  Transactions,  Part  II.,  for  1850,  I  proved  that 
those  parts  of  London  situated  near  the  river  Thames  are  somewhat  warmer  upon  the  whole 
year  than  the  country,  but  that  those  parts  of  London  which  are  situated  at  some  distance 
from  the  river  do  not  enjoy  higher  temperatures  than  those  due  to  their  latitudes.— ED.] 


CLIMATE.  257 

special  heat  of  the  trees  which  results  from  the  various  reactions  which 
take  place  in  their  tissues,  and  that  which  they  derive  from  liquids  ab- 
sorbed by  the  roots,  because  it  is  very  slight  as  compared  to  that  caused 
by  solar  or  nocturnal  radiation,  as  is  proved  by  the  maxima  and  mini- 
ma of  temperature  which  correspond  with  the  maxima  and  minima  of 
the  air,  though  occurring  at  different  hours  of  the  day.  This  special 
heat  of  trees  plays  an  important  part  in  winter,  by  preventing  a  decline 
in  temperature  which  would  be  fatal  to  them.  In  a  tree  twenty  to 
twenty-four  inches  in  diameter,  the  maximum  temperature  occurs  in 
summer  about  10  or  11  P.M.  ;  in  winter,  toward  6  P.M.  ;  whereas  in  the 
air  it  is  at  2  or  3  P.M.,  according  to  the  season.  From  this  difference  be- 
tween the  hours  of  maxima,  it  results,  as  experience  has  proved,  that  the 
temperature  of  the  air  may  be  lowered  by  some  cause,  such  as  the  pas- 
sage of  a  cloud,  a  change  in  the  direction  of  the  wind,  etc.,  and  yet  rise 
in  the  interior  of  trees,  because  of  the  heat  acquired  by  the  outer  sur- 
face, which  is  transmitted  slowly  to  the  inner  portion  of  the  tree,  owing 
to  its  non-conductibility.  The  abundance  of  forests  and  moisture  tend 
to  lower  the  temperature,  while  clearing  away  timber  and  causing 
dryness  of  atmosphere  produce  a  contrary  effect ;  the  difference  in 
some  cases  for  the  mean  temperature  of  the  year  being  as  much  as  four 
degrees  nearly. 

The  numerous  observations  taken  by  MM.  Becquerel  in  the  Loiret 
have  been  particularized  by  them  as  follows : 

1st,  in  summer,  the  mean  temperatures  of  the  air  outside  of  woods 
are  higher  than  they  are  inside. 

2d,  in,  winter,  the  reverse  is  the  case. 

3d,  the  difference  between  the  mean  annual  temperature  of  the  air  at 
several  miles  from  woodland  and  that  inside  a  wood  is  about  3°. 

The  mean  temperatures  of  the  air  in  summer  being  about  2^°  higher 
outside  than  they  are  inside  a  wood,  and  the  reverse  being  the  case  in 
winter,  it  follows  that  the  woodland  climate  is  not  so  extreme  as  that  of 
the  open  plain ;  it  partakes,  therefore,  of  the  nature  of  a  warm,  climate 
in  respect  to  temperature.  Local  conditions  modify  more  or  less  the 
general  type  of  climates.  The  greatest  local  action  is  always  exercised 
by  unevenness  of  soil.  The  mountain  chains  divide  the  surface  of  the 
earth  into  large  basins,  into  deep,  hollow,  or  circular  valleys.  These 
valleys,  often  shut  in,  as  between  ramparts,  individualize  local  climates 
(in  Greece,  for  instance,  and  in  part  of  Asia  Minor),  and  place  them  in 
special  conditions  in  reference  to  heat,  moisture,  the  transparency  of  the 

17 


258 


THE  ATMOSPHERE. 


air,  and  the  frequency  of  winds  and  storms.  After  having  studied  the 
general  condition  of  climates,  and  before  coming  to  the  poles  in  the 
course  of  this  short  geographical  review,  it  is  interesting  to  endeavor  to 
form  a  correct  idea  of  the  extreme  differences  of  temperature  through- 
out the  world. 

In  no  place  of  the  globe,  and  in  no  season,  has  the  thermometer  at  an 
elevation  of  two  or  three  yards  above  the  soil,  and  sheltered,  reached 

135°. 

In  the  open  sea  the  temperature  of  the  air  has  never  exceeded  86°. 

The  most  extreme  degree  of  cold  ever  recorded  upon  a  thermometer 
suspended  in  the  air  is  72°  below  zero. 

The  extreme  difference  in  the  temperatures  of  the  atmospheric  air  is, 
therefore,  207°. 

Comparing  together  the  most  extreme  temperatures  recorded,  Arago 
constructed  the  remarkable  table  appended.  The  places  are  given  ac- 
cording to  their  decrease  in  latitude. 


Places. 

Latitude. 

Longitude. 

Highest 
Temperature 
observed. 

Lowest 
Temperature 
observed. 

Difference. 

Melville  Island 

Deg. 
74-47  N. 

Deg. 
113-8 

Deg. 
+    60-1 

Deg. 
—54*9 

Deg. 
115*0 

PortFelix        

70-0 

94-13 

4-    70'0 

—59*4 

129-4 

Nijnei-Kolymsk  
Reikiavik 

68-32 
64-8 

158-34 
24*16 

+    72-5 

+    68  '9 

-65*0 
—  4*0 

137-5 
92  '9 

Drontheim 

63-26 

8-3 

+    83  '7 

—10*7 

74-4 

Jakoutsk 

62-2 

127*23 

-f    86-0 

—72-4 

158*4 

Abo                          ..    .  . 

60-27 

19-57 

+    95-0 

32*8 

127*8 

St  Petersburg       

59-56 

27*58 

+    90-0 

37*8 

127*8 

Upsal  

59-52 

15*18 

4-    86*0 

—  25*1 

111*1 

Stockholm  
Nijnei-Taguilsk 

59-20 
57  -56 

15*43 

57-48 

+    99*5 
+    95*0 

-28*7 
—60'7 

128*2 
•  155*7 

Kasan 

55-48 

46-47 

+   96*8 

—40-0 

136*8 

Moscow.                

55-45 

35-14 

+    94-1 

—46*7 

140'8 

Hamburg  

53-33 

7-38 

+    95*0 

—22*0 

117-0 

Berlin 

52'31 

11  '3 

+  102'7 

19*8 

122'5 

51'31 

2  '28 

+    95*0 

+  5*0 

100  "0 

Dresden  

51'4 

11*24 

+  101'8 

—  25*8 

127*6 

Brussels  

50-51 

2-1 

+    95*0 

—  6*0 

101*0 

Liege 

50*39 

3*11 

+    99*5 

—11*9 

111*4 

Lille                         

50'39 

0*4 

+    96*1 

0*4 

96*5 

Dieppe  ... 

49-49 

1*12 

+    92*3 

3*6 

95*9 

Rouen*  

49-26 

10'15 

+  100*4 

—  7'2 

107*6 

Metz 

49-7 

3*50 

+  100*6 

6*3 

106*9 

Paris 

48'50 

O'O 

+  104*0 

10*3 

114*3 

48-35 

5*2 

+    96*6 

15*3 

111*9 

Munich  (1765  feet)  
Bale                          

48-8 
47*33 

9.14 
5  15 

+    95-0 
+    93*2 

-19*8 
35*5 

114*8 
128*7 

Buda  

47'29 

16-43 

+    96*3 

8*5 

105  "3 

Tours  

47-24 

1'39 

+  100*4 

13*0 

113*4 

Dijon 

47*19 

2-49 

+      OC.1 

Quebec    ..  . 

46*49 

73*36 

+    99*5 

40  "0 

139*5 

Lausanne  (1732  feet)  
Geneva.... 

46*31 
46-12 

4-18 
3-49 

+    95*0 
•4-    97-2 

-  4*0 

—  13-n 

99*0 
nn-7 

CLIMATE. 


259 


Places. 

Latitude. 

Longitude. 

Highest 
Temperature 
observed. 

Lowest 
Temperature 
observed. 

Difference. 

St.  Bernard  (81  72  feet)... 
Gr.-Chartr.  (0660  feet)... 
Grenoble  
Turin  

Deg. 
45-50 
45-18 
45-11 
45-4 

Deg. 
4-45 
3-23 
3-24 
5-21 

Deg. 
+    67-4 
+    81-5 
+    95-0 
+    99-7 

Deg. 
-22*4 
-15-3 
-  6-9 
—  0*0 

Deg. 
89-8 
96-8 
101-9 
99*7 

Le  Puy  (2493  feet) 

45  -0 

1-33 

-j_    93  -g 

q.o 

Orange  

44-8 

2-28 

+  106  '5 

—  0-4 

106*9 

Toulouse 

43-37 

0'54 

+  104  "0 

+  4*3 

99*7 

Montpellier 

43-37 

1-32 

+  101*5 

0*4 

101*9 

Marseilles  
Perpignan  

43-18 
42-42 

3-2 
0'34 

+    98-4 
4~  101*5 

+  0-5 
+15"! 

100*7 
86*4 

Rome  

41-54 

10'7 

+  100*4 

+  19*6 

80*8 

Naples 

40'51 

11*55 

+  104*0 

+  23*0 

81*0 

Pekin   .    . 

39-54 

114-9 

+  109*6 

+  3*9 

105*7 

Lisbon  

38*42 

11'29 

+  101*8 

+  24*7 

77*1 

Palermo  

38-7 

11  '1 

+  103*5 

+32*0 

71*5 

36'5 

0"44 

+    99*5 

+  27*5 

23  '9 

84*43 

+    90*1 

+45"! 

35*0 

Vera-Cruz 

19-12 

98*29 

--)-    9g'i 

+60  '8 

35*3 

Cura9ao     .  .  . 

12'6 

71*16 

+    92*0 

+  75"0 

17*0 

Pulo-Penang  Island  
Quito  (9540  feet)  
St.  Louis  de  Marana  
Isle  of  Bourbon  

5-25 
0-14  S. 
2-31 
20-52 

97-59 
81-5 
46-36 
53-10 

+    90-0 
+    71-6 
+    91-9 
+    99-5 

+  75-9 
+42-8 
+  75-0 
+60-8 

14-1 

28-8 
16-9 
38-7 

Generally  speaking,  the  differences  between  the  highest  and  the  low- 
est temperatures  are  less  the  farther  one  travels  from  the  pole  toward 
the  equator. 

Let  us  now  deal  with  the  limits  of  climates,  the  extremity  of  the 
world,  the  icy  regions  of  the  poles. 

In  the  neighborhood  of  the  Polar  Circle  the  sea  becomes  frozen,  and 
assumes  a  special  character.  This  phenomenon  seems  to  increase  as  the 
water  gets  less  briny,  and  as  the  rotatory  movement  declines  in  rapidi- 
ty. Even  in  50°  of  latitude  pieces  of  floating  ice  are  met  with  in  the 
sea.  These  have  become  detached  from  some  more  northern  region, 
and  carried  off  by  the  currents  which  run  from  the  poles  to  the  equa- 
tor. At  55°  it  is  by  no  means  rare  to  find  the  sea-shore  strewn  with 
ice.  At  60°  the  gulfs  and  the  inland  seas  are  often  frozen  all  over.  At 
70°  the  floating  blocks  of  ice  become  very  numerous  and  very  large, 
forming  sometimes  regular  islands  as  much  as  a  half  league  in  diame- 
ter. Finally,  at  80°,  there  is  found,  as  a  rule,  fixed  ice — that  is,  ice 
which  has  become  accumulated  and  bound  together. 

These  solitary  regions  offer  a  striking  spectacle. 

The  polar  ices  are  tinted  with  the  brightest  hues,  and  seem  like 
blocks  of  precious  stones,  forming  vast  plains  and  lofty  mountains. 

The  fields  of  ice  are  often  composed  of  extensive  plainsr  perfectly 
level,  without  either  fissure,  hollow,  or  elevation.  Scoresby  saw  one 


2gO  THE  ATMOSPHERE. 

of  these  floating-fields  upon  which  a  carriage  might  have  been  driven 
for  thirty-five  leagues  without  the  slightest  interruption.  When  these 
masses  meet,  the  report  of  the  shock  is  like  a  clap  of  thunder. 

The  mountains  of  floating  ice,  as  seen  for  the  first  time  by  the  navi- 
gator who  has  made  his  way  into  the  polar  regions,  present  a  striking 
spectacle.  Dr.  Hayes,  in  his  voyage  to  the  Arctic  seas  (1860),  has  con- 
veyed to  his  readers  the  first  impression  produced  by  the  sight  of  them. 


'We  met  our  first  iceberg  the  day  before  we  reached  the  Polar 
Circle.  Hearing  the  sea  breaking  furiously  against  the  mass,  as  yet 
concealed  by  the  mist,  the  helmsman  was  upon  the  point  of  crying 
out,  'Land  ahead!'  But  almost  immediately  the  formidable  colossus 
emerged  from  the  fog,  bearing  down  upon  us,  terrible  and  threatening ; 
we  hastened  to  get  out  of  its  way.  It  formed  an  irregular  pyramid, 
about  three  hundred  feet  wide  and  one  hundred  and  fifty  feet  high ;  its 
summit  was  half  hid  in  the  mist;  but  the  latter  suddenly  lifting,  ex- 
posed to  our  gaze  a  dazzling  peak,  around  which  were  folded  light  va- 
pors. There  was  something  very  striking  in  the  indifference  of  this  gi- 
ant, which  the  waves  caressed  in  vain,  while  it  passed  on  its  way,  deaf 
to  their  charms. 

"In  Davis  Straits  we  had  to  pass  many  cruel  hours;  and  upon  one 
occasion  I  thought  that  our  last  moment  had  arrived.  We  were  run- 
ning against  the  wind,  all  sails  bent  and  a  heavy  swell  on,  when  the 
bows  gave  way,  and  all  the  sails  fell  on  to  the  deck,  nothing  remaining 
save  the  chief  sail,  which  was  flapping  violently  against  the  mast ;  and 
it  was  only  owing  to  a  miracle  of  firmness  on  the  part  of  the  helmsman 
that  we  escaped  complete  shipwreck. 

"  For  most  of  us  Greenland  was  still  a  kind  of  myth ;  for  some  days 
we  had  been  following  the  coast-line :  beyond  the  appearance  of  Disco, 
the  clouds  and  fog  had  kept  it  constantly  hidden  from  our  gaze.  But 
suddenly  it  emerged  from  its  mantle  of  mist,  and  stood  out  before  us  in 
all  its  splendor;  its  extensive  valleys,  its  noble  mountains,  its  abrupt 
and  sombre  rocks  adding  to  its  terrible  desolation. 

"  In  proportion  as  the  fog  and  mist  rolled  slowly  over  the  surface  of 
the  blue  waters,  the  mountains  of  ice  succeeded  each  other  and  defiled 
before  us  like  the  fantastic  palaces  in  a  fairy  tale.  Forgetting  that  they 
would  come  spontaneously  toward  the  region,  they  seemed  to  us  to  be 
attracted  by  an  invisible  hand  into  this  enchanted  land." 

The  ice  met  with  on  the  coasts  of  Spitzbergen  and  Greenland  is,  gen- 


CLIMATE.  261 

erally,  from  twenty  to  twenty-five .  feet  thick,  often  forming  immense 
plains,  the  limits  of  which  can  not  be  seen  from  the  topmast  of  a  vessel : 
these  are  called  the  ice-fields.  They  may  be  estimated  as  having  an  ex- 
tent of  three  hundred  to  four  hundred  square  leagues.  An  ice-field 
sometimes  presents  an  entirely  level  surface ;  at  others,  it  is  rough  and 
uneven,  with,  at  intervals,  columns  twenty  or  thirty  feet  high.  These 
columns  give  it  a  very  picturesque  aspect,  and  which  are  sometimes  of 
a  topaz  blue  tint,  sometimes  covered  with  thick  snow. 

The  undulations  of  the  water,  the  movement  of  the  waves,  or  some 
other  potent  cause,  break  up  a  field  of  ice  in  a  moment,  and  reduce  it 
into  fragments  of  1000  or  2000  square  feet.  These  fragments,  becom- 
ing separated,  come  into  collision  and  disperse ;  but  sometimes  they  are 
carried  off  by  a  rapid  current.  In  that  case,  if  they  meet  a  current  run- 
ning in  the  opposite  direction,  which  is  floating  away  large  masses  of 
ice  from  some  other  field,  these  mountains  meet  with  a  terrible  shock. 

The  icebergs,  lifted  up  out  of  the  water,  fall  the  one  on  to  the  other, 
become  covered  with  fragments  more  or  less  voluminous,  and  thus  com- 
pose regular  mountains,  with  ravines  and  indentations,  which  rise  from 
thirty  to  fifty  feet  above  the  water.  The  part  out  of  the  water  is,  as  a 
rule,  in  regard  to  the  portion  submerged  as  one  to  four ;  consequently 
the  total  height  of  these  mountains  is  from  130  to  200  feet.  Sometimes, 
too,  icebergs  100  to  130  feet  long,  which  are  very  heavy  at  their  two 
extremities,  sink  so  deep  into  the  water  that  a  vessel  may  pass  over 
them.  But  in  this  case  the  crew  is  exposed  to  the  most  fearful  risk,  as 
the  least  shock,  the  least  cause,  may  disturb  the  equilibrium  which 
keeps  the  mass  submerged,  and  if  that  cause  occurs,  the  iceberg  rises 
suddenly  and  hurls  the  vessel  into  the  air  or,  at  any  rate,  shivers  it  into 
pieces. 

In  Baffin's  Bay  there  are  mountains  of  ice  much  higher  than  in  the 
seas  of  Greenland,  some  having  been  found  to  measure  100  to  130  feet 
out  of  the  water,  which  is  equivalent  to  a  total  height  of  660  feet.  It  is 
supposed  that  these  fearful  masses  are  formed  upon  the  coasts  where 
they  shut  in  the  valleys  which  abut  upon  the  sea,  and  that  they  can  be- 
come detached.  In  summer-time  the  waters  flow  from  their  summits 
and  form  immense  cascades,  which  are  sometimes  overtaken  by  frost. 
This  is  a  majestic  spectacle,  but  it  must  be  witnessed  from  a  distance,  as 
all  of  a  sudden  these  columns  suspended  in  the  air  will  snap  short  and 
fall  into  the  sea. 

Scoresby  often  saw  ice  form  upon  the  open  sea  at  twenty  leagues  from 


262 


THE  ATMOSPHERE. 


the  shore.  As  soon  as  the  first  embryos  of  the  crystals  become  percep- 
tible, the  sea  gets  calm,  just  as  if  oil  had  been  poured  over  its  surface. 
These  crystals  soon  attain  three  or  four  inches  in  size,  and  it  is  then 
that  they  begin  to  agglomerate  if  the  cold  continues,  forming  a  sheet  of 
ice  which  soon  attains  a  thickness  of  from  eight  inches  to  a  foot. 

In  these  countries  the  density  of  sea-water  is  1/026 ;  when  still,  it 
freezes  at  28°  4.  The  water  which  has  been  concentrated  by  the  frost 
may  attain  a  density  of  1104,  in  which  case  it  will  only  congeal  at  14°; 
and  it  is  well  known  that  water  saturated  with  salt  will  not  solidify  till 
the  temperature  is  less  than  5°. 


Pig.  65.— The  last  human  dwelling-places.    Esquimaux  of  the  Polar  Regions. 

These  desolate  regions  where  mercury  freezes  in  the  open  air,  are 
nevertheless  inhabited  by  the  Esquimaux,  who  are  the  remotest  inhabit- 
ants to  the  north,  living  as  they  do  in  the  79th  degree  of  latitude.  Dr. 
Kane  visited,  in  1853,  two  of  their  villages  upon  the  Greenland  coast  of 
Smith's  Straits,  at  11°  from  the  pole.  These  villages  are  called  Etah  and 
Peterovik,  and  the  capital  of  the  country  is  Upernavik,  which  was  vis- 
ited in  1861  by  Dr.  Hayes.  An  idea  of  the  place  in  which  these  people 
(from  whom  America  is  descended)  dwell  may  be  gathered  from  Fig. 
55.  The  huts  are  constructed  upon  landings  with  blocks  of  snow  cut 
into  the  shape  of  domes.  The  entrance  is  by  a  circular  and  very  small 


Fig.  56.— Ice  at  the  Pole. 


CLIMATE.  265 

opening,  and  light  is  admitted  by  means  of  a  small  window,  in  which  a 
diaphanous  piece  of  snow  serves  the  purpose  of  a  pane  of  glass. 

The  point  nearest  to  the  pole  as  yet  reached  is  six  degrees  and  a 
quarter  (lat.  83°  45'),  which  is  only  about  170  leagues  from  it.  Parry 
and  Sir  James  Boss  approached  thus  far  in  1826.  The  ill-fated  Frank- 
lin did  not  pass  beyond  77°.  Dr.  Hayes  navigated  in  the  polar  sea  as 
far  as  81°  40'  in  the  month  of  May,  1861. 

Let  us  conclude  this  general  view  of  climate  by  remarking  that  the 
last  isothermal  line,  clearly  established  by  observations,  is  that  of  +5°, 
which  descends  to  the  north  of  America,  re-ascends  to  the  north  of  Baf- 
fin's Bay,  crosses  the  80th  degree  of  latitude,  afterward  extends  to  de- 
gree 70,  and  even  to  degree  65.  This  line  forms  two  bends,  in  each  of 
which  there  is  recorded  an  increase  of  cold.  It  is  not  at  the  pole  itself 
that  the  mean  temperature  is  lowest,  but  on  either  side  of  it.  There  are 
thus  what  may  be  termed  two  poles  of  cold,  one  situated  to  the  north 
of  the  Asiatic  continent,  not  far  from  the  archipelago  known  as  New  Si- 
beria, where  the  mean  temperature  is  +  l°-4;  the  other  to  the  north  of 
the  American  continent,  in  the  western  isles  of  the  Polar  Archipelago, 
and  its  temperature  appears  to  be  —  20-2.  It  is  probable  that  two  anal- 
ogous poles  of  cold  exist  as  well  in  the  frozen  Antarctic  Ocean.  As  to 
the  North  Pole  itself,  the  early  calculations  of  Plana,  the  mathematician, 
of  the  geometer  Lambert,  and  of  the  astronomer  Halley,  as  well  as  those 
of  my  regretted  friend  Gustave  Lambert,  establish  conclusively  the  fact 
that  the  cold  is  much  less  intense  there.  As  to  our  pole  (taking  into 
account  refraction),  the  sun  rises  in  the  beginning  of  March,  mounts 
slowly,  skimming  the  horizon,  and  follows  a  spiral  line  which  takes  a 
greater  elevation  each  successive  day.  It  does  not  again  set  until  the 
end  of  September.  On  the  21st  of  June  it  attains  its  greatest  elevation. 
The  maximum  of  heat  prevails  in  July  and  August.  From  these  calcu- 
lations and  the  direct  observations  of  navigators  who  have  penetrated 
the  nearest,  it  follows  that  the  sea  is  not  frozen  at  the  North  Pole  itself. 


BOOK  FOURTH. 

THE  WIND. 


THE  WIND  AND  ITS  CAUSES.  269 


CHAPTER  I 

THE  WIND  AND  ITS  CAUSES:  GENERAL  CIRCULATION  OF.  THE  ATMOS- 
PHERE—  THE  REGULAR  AND  PERIODICAL  WINDS — TRADE- WINDS — 
THE  MONSOON — BREEZES. 

WE  now  come  to  the  study  of  the  great  currents  of  the  atmosphere, 
which  are  themselves  the  incessant  manifestation  of  the  sun's  action 
upon  our  planet.  Without  the  wind  the  atmosphere  would  remain 
motionless  about  the  globe ;  heavy,  cold,  deadened,  enveloping  the  earth 
in  a  regular  pall,  never  agitated  by  a  breath  of  air,  a  receptacle  for  ev- 
ery kind  of  miasma — poisonous  and  deleterious.  By  its  agency  an  im- 
mense circulation  is  established  from  one  end  of  the  world  to  the  other, 
renewing  all  the  strata,  sweeping  away  unhealthy  exhalations,  substi- 
tuting for  oppressive  heat  a  refreshing  coolness,  or  replacing  the  period 
of  frost  by  the  warmth  of  spring. 

What  is  wind?  In  this  section  of  our  work,  and  in  the  succeeding 
one,  which  deals  in  clouds  and  rain,  we  take  in  hand  the  general  data 
of  meteorology ;  for  the  currents  of  air  on  the  one  hand,  and  water  on 
the  other,  cause  the  varying  meteorological  conditions  of  the  seasons 
and  of  years.  It  is  on  this  head  particularly  that  we  have  an  exact  base 
for  our  knowledge,  and  that  we  are  in  a  position  to  consider  the  general 
mechanism  of  this  vast  factory,  which  distributes  benefits  and  disasters 
over  the  earth,  and  among  the  people  which  inhabit  it.  Meteorology 
will  not  be  able  to  hold  her  own  with  her  elder  sister,  Astronomy — that 
is,  to  be  precise  in  respect  to  ascertained  principles,  and  to  enable  science 
to  announce  the  movements  to  the  atmosphere,  the  winds,  the  rains,  the 
droughts,  and  the  tempests,  as  the  latter  announces  the  movement  of  the 
stars — until  we  are  able  to  embrace,  in  one  glance,  the  general  circula- 
tion which  is  constantly  going  on  all  over  the  globe,  and  which  gives 
rise  to  divergencies  which  occur  in  different  seasons  and  at  different 
places. 

What  is  the  wind  ? 

It  is  neither  more  nor  less  than  a  certain  quantity  of  air  set  in  motion 
by  a  change  in  the  equilibrium  of  the  atmosphere.  The  varying  tempera- 
tures to  which  the  different  parts  of  the  atmosphere  are  constantly  ex- 


270  THE  ATMOSPHERE. 

posed  rarefy  each  of  these  parts  in  a  different  manner.  When  air  is 
heated  its  weight  diminishes,  and  it  has  a  tendency  to  rise;  whereas 
colder  air  becomes  heavier,  and  flows  to  supply  the  place  of  the  heated 
air,  and,  in  its  passage  toward  the  re-establishment  of  an  equilibrium, 
will  cause  a  current  of  air  which  is  termed  wind,  and  which  will  con- 
tinue till  an  equilibrium  is  restored. 

Let  us  suppose,  for  a  moment,  that  the  atmosphere  is  perfectly  calm 
everywhere.  A  cloud  passes  over  the  sun,  the  air  that  is  situated  in 
a  line  with  its  passage  is  rendered  cooler,  undergoes  condensation,  and 
becomes  denser ;  this  air  seeks  an  equilibrium ;  a  primary  movement 
will  take  place  in  the  direction  of  the  cloud,  and  here  we  have  a  current 
of  fresh  air,  the  tendency  of  which  will  be  to  occupy,  as  quickly  as  pos- 
sible, the  place  of  the  hotter  and  more  dilated  air  which  is  next  to  it. 
Suppose  that  the  sun,  shining  in  a  clear  sky,  remains  motionless  above 
our  heads.  The  air  situated  immediately  underneath  will  become  heat- 
ed more  rapidly  than  that  which  receives  its  rays  obliquely.  Becoming 
dilated,  it  will  rise  toward  the  less  dense  aerial  regions,  the  air  which  is 
contiguous  to  it  will  force  itself  into  its  place,  and  thus  another  current 
of  air  is  established. 

The  great  atmospheric  currents,  the  winds,  general  and  special,  are 
nothing  else  than  this  unceasing  pursuit  toward  an  equilibrium  which 
is  perpetually  being  destroyed  by  the  various  influences  of  the  sun. 
This  will  be  seen  by  applying  to  the  entire  surface  of  the  globe  the  in- 
stance cited  above.  In  what  way  are  two  contiguous  parts  of  the  at- 
mosphere affected  if  they  become  heated  in  unequal  proportions  ?  Near 
the  equator,  the  sun,  as  its  rays  reach  the  earth  in  a  perpendicular  or 
nearly  vertical  direction,  causes  a  temperature  which  is  constantly  high- 
er than  at  other  points  of  the  globe.  It  follows  from  this  that  two  in- 
ferior currents  must  flow  from  the  two  hemispheres  toward  the  equator. 

The  air,  which  is  very  heated  in  the  equatorial  zone,  rises  in  a  mass 
toward  the  higher  regions  of  the  atmosphere.  Having  reached  an  ele- 
vation of  several  miles  (but  which  we  are  unable  to  calculate  exactly), 
the  ascending  mass  breaks  into  two,  which  pass  away  in  the  direction 
of  the  two  poles. 

This  ascensional  movement  thus  produced  gives  rise  to  a  rush  of  air 
from  the  two  sides  of  the  torrid  zones,  and  two  other  masses,  skimming 
the  surface  of  the  ground,  make  their  way  from  the  temperate  regions 
toward  this  line.  Thus  we  discover  all  over  the  earth  a  double  aerial 
circuit. 


THE  WIND  AND  ITS  CAUSES.  271 

Let  us  first  take  the  northern  circuit.  A  current  of  air,  starting  from 
the  tropical  regions,  proceeds  toward  the  equator.  Situated  in  the  lower 
regions  of  the  atmosphere,  and  upon  the  surface  of  the  globe,  this  cur- 
rent comes  directly  beneath  our  observation,  and  it  constitutes  the  trade- 
winds  of  the  northern  hemisphere.  When  within  a  short  distance  of 
the  equator,  a  distance  which  varies  with  the  seasons,  it  suddenly  rises, 
and,  when  it  has  reached  a  certain  level,  takes  a  directly,  horizontal 
march  toward  the  pole,  gradually  descending  toward  the  surface  as  its 
distance  from  the  equator  increases.  Maury  termed  this  kind  of  cur- 
rent the  upper  anti-trade-wind. 

If  it  stopped  there  the  current  would  not  be  complete;  the  trade- 
winds  and  the  anti-trade-winds,  connected  with  each  other  by  the  as- 
cending branch  of  the  equatorial  region,  are  not,  as  yet,  united  on  the 
northern  side.  If  the  earth  were  motionless,  and  the  whole  of  its  sur- 
face received  light  at  the  same  time ;  if,  moreover,  its  surface  was  uni- 
versally homogeneous,  the  meeting  of  the  two  hrizontal  branches  would, 
no  doubt,  take  place  toward  the  north,  as  it  does  toward  the  south,  ex- 
cepting, of  course,  the  reversal  of  the  direction  of  the  movement.  The 
upper  anti-trade-wind  would  incline  toward  the  ground,  so  as  to  join 
the  trade-wind,  and  the  circulation  of  the  atmosphere  would  be  almost 
comprised  within  heights  of  an  inconsiderable  elevation.  Let  us  re- 
mark, however,  that  as  the  first  origin  of  the  movement  is  at  the  equa- 
tor, the  movement  will  be  regular  there,  like  the  cause  which  produces 
it.  The  trade-winds  and  the  anti-trade-winds  will  themselves  partici- 
pate of  this  regularity  in  the  neighborhood  of  the  equinoctial  line ;  but 
the  farther  one  recedes  from  this  line  the  less  directly  will  the  motive 
force  act.  The  descending  mass  will,  therefore,  be  more  diffuse,  less 
compact,  and  less  fixed  in  its  quantity  than  the  ascending  mass.  Its 
mean  position  will  depend  upon  the  mean  activity  of  the  equatorial 
draught,  and  upon  the  height  to  which  the  trade-winds  reach.  This 
height  is  itself  dependent  upon  the  law  of  the  decrease  in  the  tempera- 
ture, according  to  the  altitude.  It  may  vary  with  the  seasons,  and  has 
probably  not  been  the  same  in  all  ages  of  the  world. 

The  southern  circuit  is  rather  more  extensive  than  the  northern ;  it 
encroaches  upon  the  northern  hemisphere,  upon  the  surface  of  the  At- 
lantic, and  in  summer  this  encroachment  is  more  marked  than  is  the 
case  in  winter. 

Circulation,  regular  as  it  may  be,  can  not  take  place  in  the  midst  of 
an  atmosphere  always  in  motion  like  ours,  without  reacting  upon  the 


272 


THE  ATMOSPHERE. 


part  which  is  not  directly  comprehended  in  the  movement.  The  de- 
crease of  the  temperature  extends  also  toward  the  poles,  and  atmos- 
pheric movements  are  the  forced  consequences  in  these  high  latitudes. 
Two  leading  circumstances  cause  the  aerial  currents  to  travel  out  of 
the  limits  comprised  in  the  above  circuits,  and  give  rise  to  two  second- 
ary circuits  (N'  and  S') ;  these  are  the  rotation  of  the  earth  on  its  axis 
around  the  sun,  and  the  division  of  land  and  water  over  the  globe. 
Summer 


South  Pole 


florlh.  Pole 


Fig.  57.— Section  of  the  atmosphere,  showing  its  general  circulation. 

The  earth  turns  upon  its  axis  in  the  direction  of  west  to  east.  In 
virtue  of  this  rotation,  every  point  of  it  completes  a  revolution  in  the 
same  period  of  twenty-four  hours ;  but  in  this  interval  of  time  all  parts 
do  not  traverse  the  same  distance  or  move  at  the  same  rate  of  speed. 
At  the  equator  the  speed  is  about  416  leagues  an  hourj  in  the  latitude 
of  Paris  it  is  273 ;  at  degree  56  it  is  231 — as  at  Edinburgh,  for  instance ; 
at  the  poles  it  is  nothing. 

The  air  which  seems  to  us  to  be  in  repose  at  Paris  is,  in  reality,  mov- 
ing there  at  the  rate  of  273  leagues  an  hour.  Let  us  imagine  this  air 
transported  to  the  latitude  of  56°  without  any  change  in  its  velocity ;  it 
will  continue  to  travel  273  leagues  per  hour.  As  each  point  in  latitude 
56°  travels  at  231  leagues  per  hour,  the  air  will  gain  upon  the  ground, 
in  an  easterly  direction,  at  the  rate  of  forty-two  leagues  an  hour!  which 
would  constitute  a  hurricane.  The  reverse  would  be  the  case  if  a  mass 
of  air,  relatively  still,  in  parallel  56°,  were  suddenly  transported  into 


THE  WIND  AND  ITS  CAUSES.  273 

parallel  49°.  This  air  would  appear  to  us  to  be  traveling  from  east  to 
west  at  the  rate  of  forty-two  leagues  per  hour. 

In  reality,  these  passages  of  air  from  one  parallel  to  another  always 
take  place  gradually,  and,  during  their  transition,  resisting  causes  of  va- 
rious kinds  tend  to  equalize  their  velocity.  The  lessened  differences 
none  the  less  continue  in  operation,  and,  as  the  size  of  the  parallels  of 
latitude  diminishes  the  more  rapidly  on  approaching  the  poles,  the  ef- 
fects pointed  out  above  become  more  and  more  pronounced  as  they  oc- 
cur in  higher  latitudes.  Many  tempests  are  derived  from  this  cause. 

The  influence  of  the  earth's  rotation  upon  the  direction  of  the  trade- 
winds  is  as  follows : 

Take,  first,  the  trade-winds  of  the  northern  circuit.  We  have  sup- 
posed that  they  move  from  north  to  south  toward  the  equator.  During 
this  movement  they  pass  gradually  by  the  parallels,  whose  diameters, 
and  consequently  whose  speed,  progressively  increase.  If  their  abso- 
lute velocity  does  not  diminish,  they  will  apparently  move  toward  the 
west,  and  their  seeming  direction  will  be  from  north-east  to  south-west, 
which  is,  in  fact,  somewhere  about  the  direction  of  the  trade-winds  fh 
the  northern  hemisphere.  A  like  result  follows  in  the  case  of  the 
southern  trade-winds,  which  also  seem  to  retrograde  toward  the  west; 
but  as  these  winds  travel  from  south  to  north  toward  the  equator,  their 
apparent  direction  will  be  from  the  south-east  toward  the  north-west, 
which  is,  in  fact,  the  general  direction  of  the  trade-winds  in  the  south- 
ern hemisphere. 

When  the  ascending  mass,  having  reached  a  certain  height,  divides 
into  two  horizontal  masses,  which  form  the  upper  or  anti-trade-winds, 
flowing  from  the  equator  toward  the  poles,  and,  little  by  little,  travel 
past  parallels  the  speed  of  which  is  successively  less  and  less,  they  soon 
take  an  easterly  bend  in  these  parallels,  and  their  apparent  direction  is 
toward  the  north-east.  When  they  have  arrived  at  a  certain  distance 
from  the  neighborhood  of  the  tropics,  they  descend  toward  the  earth ; 
then  is  reproduced  the  phenomenon  noticeable  in  the  ascending  mass; 
the  anti-trade-winds  find  their  way  with  the  velocity  which  they  have 
acquired  and  their  easterly  tendency ;  the  inclination  of  their  speed  in  a 
vertical  direction  renders  this  speed  less  apparent,  and  we  meet  with 
two  new  regions  in  these  latitudes,  called  tropical  calms.  In  moving 
from  the  equator  toward  the  North  Pole,  we  thus  encounter:  1st.  The 
region  of  equatorial  calms ;  2d.  The  north-easterly  trade-winds ;  3d.  The 
tropical  calms ;  and  4th,  beyond  these,  winds  varying  from  south-west 

18 


274  THE  ATMOSPHERE. 

to  north-west.     The  same  series  is  met  with  in  the  southern  hemi- 
sphere. 

In  a  word,  we  find  that  there  are  in  each  hemisphere  two  circuits 
which  have  as  a  common  basis  the  ascending  equatorial  mass.  The 
first,  a  direct  circuit,  is  generally  limited  to  the  intertropical  regions;  the 
second,  a  derived  circuit,  is,  in  reality,  only  a  prolonged  arm  of  the  first, 
and  extends  from  the  tropics  to  a  varying  distance  from  the  poles. 
These  two  circuits  are  distinguished  from  each  other  by  essential  char- 
acteristics ensuing  from  their  different  positions  in  the  atmosphere. 

The  direct  circuit  spreads  upward.  While  the  trade-winds  skim  the 
ground,  the  anti-trade-winds  circulate  in  very  lofty  regions  of  the  air. 
The  distance  which  separates  these  two  currents,  joined  to  the  regulari- 
ty of  their  movements,  prevents  them  from  encroaching  upon  each  oth- 
er or  influencing  each  other's  progress.  This  does  not  hold  good  of  the 
derived  circuit.  The  prolonged  arm  of  the  anti-trade-winds  there  be- 
comes superficial.  It  sweeps  along  the  ground;  and  so,  also,  does  the 
returning  current.  Both,  therefore,  are  upon  the  same  level,  simply 
contiguous,  and  separated  only  by  the  action  of  the  earth's  rotation. 
There  are  points  at  which  these  currents  come  together;  and  their  dif- 
ferent qualities  cause  numerous,  and  sometimes  disastrous,  atmospheric 
disturbances.  Their  beds  get  shifted  over  the  surface  of  the  globe,  and 
the  succession  of  one  after  another  in  the  same  place  produces  sudden 
variations  in  the  state  of  the  sky.  To  avoid  confusion,  the  branch  of 
the  upper  anti-trade-winds  which  is  prolonged  into  the  derived  circuit 
is  termed  the  equatorial  current,  and  the  back  current  in  the  same  cir- 
cuit is  called  the  polar  current. 

This  general  circulation  of  the  atmosphere  is  influenced  to  a  certain 
extent  by  the  seasons. 

At  the  end  of  our  summer  the  regions  about  the  North  Pole  have  for 
several  months  had  days  without  any  nights;  the  temperature  there 
has  become  perceptibly  milder  and  the  air  rarefied.  To  days  without 
nights  soon  succeed  nights  without  days,  accompanied  by  excessive 
cold ;  the  air  becomes  contracted,  and  draws  in  a  fresh  supp'ly  to  fill  up 
the  vacancy  caused  by  this  contraction.  Each  of  these  changes  in  our 
hemisphere  corresponds  with  an  exactly  reverse  change  in  the  other 
hemisphere ;  there  is,  therefore,  a  general  translation  each  year  of  the 
atmosphere  of  the  northern  hemisphere  into  the  southern,  and  vice 
versa. 

The  rush  of  air  toward  the  North  Pole  during  winter  is  brought 


THE  WIND  AND  ITS  CAUSES.  275 

about  by  the  equatorial  currents,  which  then  acquire  a  very  large  vol- 
ume. The  perturbations  increase  there  in  the  same  proportions:  it  is 
the  season  of  tempests.  As  the  sun  makes  its  way  back  to  us,  and  our 
atmosphere  becomes  heated  and  dilates,  the  equatorial  current  slackens 
its  speed  and  reaches  lower  latitudes.  On  the  other  hand,  the  polar 
currents  become  more  active ;  but  as  they  are  diffused  over  the  surface 
of  Asia  and  of  Europe,  their  speed  is  rarely  very  great,  and  summer  is 
the  calm  season  in  our  hemisphere.  The  atmospheric  disturbances  at 
this  season  never  extend  very  far,  and  their  local  gravity  is  due  to  elec- 
trical phenomena  of  a  special  nature:  it  is  the  season  of  thunder-storms. 

The  equatorial  currents  take,  at  their  polar  extremities,  a  direction 
parallel  to  the  equator,  and  march  from  west  to  east.  Notwithstanding 
their  variations,  both  in  volume  and  intensity,  it  is  easy  to  understand 
that  they  cause  the  atmosphere  at  the  poles  to  make  a  continuous  rota- 
tory movement  in  the  same  direction  as  the  earth.^ 

For  many  centuries  the  trade-winds  were  an  enigma,  both  to  meteor- 
ologists and  to  navigators.  Halley  and  Hadley  first  suggested  the  ex- 
planation which  has  been  developed,  and  which  contemporary  research 
has  modified  in  the  course  of  the  last  century. 

Between  the  two  trade  -  winds  there  are  two  zones ;  these  are  the 
zones  of  equatorial  calms.  These  calm  regions  occupy  very  different 
positions  at  the  close  of  winter  to  what  they  do  at  the  end  of  summer; 
they  follow,  in  fact,  but  at  a  distance,  the  progress  of  the  sun  between 
the  tropics.  They  never  cross  the  equator  .upon  the  surface  of  the  At- 
lantic. In  February  and  March,  months  when  they  approach  nearest  to 
it,  the  north-easterly  trade-winds  stop  at  about  4°  north  latitude ;  in  Au- 
gust and  September,  the  months  in  which  they  are  farthest,  away  from 
it,  the  same  trade-winds  stop  at  about  11°.  When  a  vessel  sailing  in 
the  Atlantic  approaches  the  equator,  the  crew  begin  to  feel  anxious,  for 
they  know  that  the  favorable  wind  which  has  brought  them  thus  far 
will  gradually  fail,  and  finally  disappear  altogether.  The  waters  extend 
around  them  like  a  vast  sheet  of  ice,  and  the  ship  is,  so  to  speak,  nail- 
ed to  the  limpid  crystal.  The  solar  rays  fall  vertically  upon  the  deck. 

The  sun  which,  twice  in  the  course  of  the  year,  pours  down  its  rays 
perpendicularly  upon  these  regions,  never  recedes  far  enough  for  any 
thing  like  coolness  to  ensue.  The  heated  atmosphere  becomes  so  light 
that  it  is  continually  ascending.  There  evaporates  also  from  the  Atlan- 
tic and  the  Pacific  Oceans  an  immense  quantity  of  water,  which  becomes 
diffused  and  mixed  with  the  heated  air,  and  ascends  with  it ;  but  as  the 


276  THE  ATMOSPHERE. 

air  ascends  to  the  lofty  regions  it  gradually  cools,  sometimes  very  sud- 
denly, so  that  a  great  part  of  the  water  which  had  accompanied  it  is 
transformed  into  drops.  These  sudden  changes  produce  passing  tem- 
pests, which  are  frequent  in  the  equinoctial  regions. 

We  have  seen  that,  as  the  wind  approaches  the  temperate  zones,  upon 
which  it  will  descend  and  become  converted  into  surface  currents,  the 
upper  current  encounters  strata  of  air,  the  speed  of  which  in  regard  to 
the  diurnal  movement  is  at  a  minimum.  It  follows  that  the  return  of 
the  trade-winds  gives  rise  in  the  temperate  zones  to  a  wind  which  blows 
from  south-wes't  in  the  northern  hemisphere,  and  from  north-west  in  the 
southern  hemisphere.  Thus,  in  France,  the  wind  blows  oftener  from 
the  south-west  than  from  any  other  direction.  At  the  time  of  the  dis- 
cussions upon  the  real  movement  of  the  earth,  the  followers  of  Coperni- 
cus adduced  the  trade- winds  as  a  proof  of  the  diurnal  rotatory  move- 
ment, from  west  to  east.  This  was  quite  an  illusion  on  their  part.  Car- 
ried by  the  movement  of  the  globe,  the  observer  would,  had  such  been 
the  case,  have  quitted  the  air  of  the  atmosphere,  which  would,  under 
those  circumstances,  have  seemed  to  give  rise  to  a  wind  blowing  in  a 
contrary  direction,  viz.,  from  east  to  west.  But  we  have  seen  that  it  is 
the  combination  of  different  rates  of  speed,  on  the  one  hand,  the  strata 
of  air  which  are  displaced  by  the  differences  of  temperature  in  the  vari- 
ous parts  of  the  globe ;  and  on  the  other  hand,  the  atmospheric  strata 
which  are  brought  under  the  influence  of  the  diurnal  movement,  which, 
in  reality,  produce  the  trade- winds.  The  theory  of  the  motion  of  the 
earth  does  not  require  this  pretended  meteorological  proof. 

The  existence  of  the  upper  counter-current  has  been  ascertained  di- 
rectly by  Captain  Basil  Hall,  who  observed  that  in  the  region  of  the 
trade-winds  very  high  clouds  are  continually  sailing  in  an  opposite  di- 
rection to  that  followed  by  the  wind  beneath.  The  same  traveler  re- 
marked upon  the  summit  of  Teneriffe  in  August,  1829,  a  south-westerly 
wind;  that  is  to  say,  a  wind  of  a  diametrically  opposite  direction  to  the 
trade-wind  which  was  blowing  upon  the  surface  of  the  ground.  When 
Humboldt  ascended  the  same  mountain  in  1799,  a  very  strong  westerly 
wind  was  blowing  upon  the  peak. 

Another  proof  of  the  existence  of  this  same  counter-current  of  the 
trade-winds  may  be  deduced  from  the  fact  of  dust  emitted  by  the  vol- 
cano in  St.  Vincent  Island  falling  upon  Barbados. 

During  the  evening  of  April  30,  1812,  explosions  resembling  the  dis- 
charge of  heavy  pieces  of  artillery  were  audible  at  Barbados ;  the  gar- 


THE  WIND  AND  ITS  CAUSES.  277 

rison  of  the  Chateau  St.  Anne  remained  under  arms  all  night.  On  the 
following  morning  the  horizon  of  the  sea,  to  the  east,  was  clear  and 
well-defined,  but  just  above  it  was  seen  a  black  cloud  which  already 
covered  the  rest  of  the  sky,  and  which,  soon  after,  spread  over  that  part 
where  the  light  of  day  was  beginning  to  break.  The  obscurity  became 
so  intense  that  persons  sitting  in  a  room  were  unable  to  distinguish  the 
window,  and  in  the  open  air  trees  and  houses,  and  even  a  white  hand- 
kerchief held  up  at  a  distance  of  six  inches  before  the  eyes,  became  in- 
visible. This  phenomenon  was  caused  by  the  fall  of  a  large  quantity 
of  volcanic  ashes,  emitted  by  a  volcano  in  the  Island  of  St.  Vincent. 
This  new  kind  of  rain,  and  the  profound  obscurity  which  accompanied 
it,  did  not  entirely  cease  until  nearly  one  o'clock.  The  trees,  whose 
timber  bends  readily,  bent  beneath  its  weight,  and  the  crash  of  the 
limbs  of  other  trees  as  they  snapped  off  short  was  in  striking  contrast 
to  the  complete  calm  of  the  atmosphere;  the  sugar-canes  were  pros- 
trated upon  the  ground,  and  the  whole  island  was  covered  with  a  layer 
of  greenish  ashes  to  a  depth  of  one  inch. 

St.  Vincent  is  fifty  miles  nearly  due  west  of  the  Barbados,  and  the 
volcano  there  had  shot  this  immense  mass  of  ashes  to  the  height  at 
which  the  upper  current  was  traveling — -a  current  which  was  itself  suffi- 
ciently strong  to  transport  the  mass. 

Halley  was  the  first  to  affirm  the  existence  of  the  upper  trade-winds 
as  a  consequence  of  the  ordinary  trade-winds.  Though  he  advanced  no 
direct  proof  of  the  fact,  he  assured  himself  of  its  truth  by  the  almost  in- 
stantaneous rotation  of  the  wind  in  opposite  directions,  when  the  polar 
limits  of  the  trade-winds  are  passed.  In  his  opinion,  as  in  that  of  all 
meteorologists  of  the  present  day,  the  equatorial  south-west  current 
which  prevails  in  the  mean  latitudes  of  our  hemisphere  is,  in  reality, 
only  a  continuation  of  part  of  our  upper  trade-winds  on  their  return 
journey. 

The  higher  branch  of  the  intertropical  circuit  is,  at  its  equatorial  ori- 
gin, at  such  a  height  that  it  has  been  impossible  to  ascertain  its  exist- 
ence with  precision,  even  by  climbing  the  loftiest  peaks  of  the  Cordil- 
leras in  the  neighborhood  of  the  region  of  calms.  But,  as  this  branch 
gradually  descends  toward  the  surface  of  the  globe,  in  proportion  as  it 
approaches  the  tropics,  and  as,  moreover,  its  course  lies  through  colder 
regions,  some  few  clouds  appear  in  the  air  which  it  carries  in  its, train. 
These  serve  as  so  many  proofs  of  the  direction  which  it  takes. 

The  existence  of  trade-winds  was  ascertained  during  the  first  voyage 


278  THE  ATMOSPHERE. 

made  by  Christopher  Columbus.  The  regular  winds,  which  impelled 
that  adventurous  navigator  along  the  new  route  by  which  he  expected 
to  reach  India,  excited  the  fears  of  his  associates,  who  doubted  the  pos- 
sibility of  getting  back  to  Europe.  Had  Columbus,  after  the  discovery 
of  the  New  World  which  he  alighted  upon  when  he  imagined  that  he 
had  reached  India,  not  taken  pains  to  avoid  the  trade-winds,  by  steering 
to  the  north  before  he  turned  westward,  he  would  assuredly  never  have 
found  his  way  back  to  Spain.  With  his  vessels  both  ill  provided  with 
food  and  defective  in  construction,  he  and  his  crews  Would  have  perish- 
ed of  hunger  in  the  vast  regions  of  the  trade-winds.  It  is  upon  the 
struggle  between  these  two  currents,  upon  the  point  at  which  the  upper 
current  descends  to  the  surface,  and  upon  their  reciprocal  mingling,  that 
depend  the  most  important  of  atmospheric  variations,  the  changes  of 
temperature  in  the  strata  of  air,  the  precipitation  of  aqueous  vapor,  and 
even,  as  Dove  has  shown,  the  varying  shape  and  form  which  clouds 
take.  The  shape  of  the  clouds,  which  lends  so  much  charm  to  our 
landscape,  indicates  to  us  what  is  going  on  in  the  higher  regions  of  the 
atmosphere.  When  the  air  is  calm  the  clouds  delineate  upon  the  sky 
on  a  warm  summer  day  "  the  projected  shape  "  of  the  soil,  the  caloric  of 
which  radiates  freely  toward  space. 

In  the  great  ocean  and  the  Atlantic  the  trade-winds  extend  nearly  to 
the  tropics ;  but  in  the  Indian  Ocean  the  presence  of  land  prevents  the 
regular  or  the  trade- winds  from  setting  in;  whereas  in  the  southern 
hemisphere,  at  a  certain1  distance  from  land,  the  south-east  trade-winds 
prevail  almost  uninterruptedly.  In  the  northern  hemisphere  of  the  In- 
dian Ocean  there  prevails  a  south-west  wind,  blowing  toward  the  penin- 
sula of  Hindoostan,  to  the  north  of  India  and  China,  from  April  till 
October;  and  from  October  to  April  the  prevailing  wind  is,  on  the 
contrary,  from  the  north-east.  These  are  the  monsoons  of  the  Indian 
Ocean.  This  word  is  derived  from  the  Malay  moussin,' which  signifies 
season.  Thus,  during  the  summer  of  our  hemisphere,  when  the  sun 
has  a  north  declination,  it  is  the  south-west  monsoon  which  prevails; 
whereas  in  our  winter,  when  the  sun  has  a  south  declination,  the  mon- 
soon is  the  north-east.  These  winds  penetrate  into  the  interior  of  con- 
tinents, where  they  are  influenced  by  the  shape  of  the  land.  The 
mountain  chains  generally  tend  to  attract  the  gaseous  masses  in  their 
direction.  The  explanation  of  these  periodical  winds  is  this :  In  Jan- 
uary the  temperature  of  South  Africa  is  at  its  maximum,  that  of  Asia  at 
its  minimum.  The  northern  portion  of  the  Indian  Ocean  is  hotter  than 


THE  WIND  AND  ITS  CAUSES.  279 

the  continent,  but  not  so  hot  as  the  southern  part  of  the  same  ocean  at 
an  equal  latitude.  We  find,  then,  in  each  hemisphere,  easterly  winds 
blowing  toward  the  hottest  points.  From  October  to  April  the  south- 
east trade-winds  prevail  in  the  southern  hemisphere;  the  north-east 
trade-winds  are  blowing  in  the  northern  hemisphere,  and  are  termed 
the  north-east  monsoon.  Between  the  two  is  the  region  of  calms. 
When  the  sun  advances  toward  the  north,  the  temperature  of  the  conti- 
nent and  that  of  the  sea  become  more  or  less  equalized ;  thus,  about 
the  period  of  the  spring  equinox,  there  are  no  prevailing  winds  in 
the  northern  hemisphere,  but  varying  winds,  which  alternate  between 
dead  calms  and  hurricanes ;  whereas  the  south-east  monsoon  prevails 
throughout  the  year  in  the  southern  hemisphere.  As  the  north  decli- 
nation of  the  sun  increases,  the  temperature  of  Asia  rises  above  that  of 
the  sea;  whereas  it  declines  below  it  in  New  Holland  and  South  Africa. 
The  relative  positions  of  the  two  continents,  the  differences  in  the  tem- 
perature which  are  most  marked,  and  the  rotatory  movement  of  the 
earth,  thus  create  a  current  from  the  south-west — a  monsoon  which  pre- 
vails from  April  to  October.  Thus,  whereas  in  the  southern  hemi- 
sphere the  trade- winds  from  the  south-east  prevail  throughout  the  year, 
the  north-east  monsoon  in  winter  and  that  from  the  south-west  in  sum- 
mer are  met  with  to  the  north  of  the  equator. 

Thus  are  indicated  in  a  brief  manner  the  general  directions  of  these 
winds.  So  far  back  as  any  records  exist,  they  facilitated  the  communi- 
cations which  were  then  so  frequent  between  India  and  Egypt.  Upon 
the  decadence  of  that  empire  these  relations  ceased,  and  the  tradition  of 
these  winds  was  lost ;  for,  otherwise,  Nearchus  would  not  have  been  so 
long  on  his  voyage  from  the  mouths  of  the  Indus  to  the  extreme  end 
of  the  Persian  Gulf. 

In  many  places  periodical  winds  are  met  with  which  alternate  with 
the  seasons,  and  which  are  influenced  by  the  shape  of  the  coast-line ; 
thus,  for  instance,  in  Brazil  there  is  a  north-east  monsoon  in  spring  and 
a  south-west  monsoon  in  autumn.  The  Mediterranean  has  its  mon- 
soons, known  to  the  ancients,  who  indicated  their  sense  of  dependence 
upon  the  winds  by  the  term  etesian  winds  (from  tVoc,  year  or  season). 
To  the  south  of  the  Mediterranean  basin  the  vast  desert  of  Sahara  ex- 
tends. Devoid  of  water,  made  up  merely  of  sand  or  conglomerated 
pebbles,  it  becomes  very  heated  under  the  influence  of  an  almost  vertic- 
al sun ;  whereas  the  Mediterranean  preserves  its  ordinary  temperature. 
Thus,  in  summer,  the  air  rises  above  the  desert  of  Sahara  with  great  ra- 


280  THE  ATMOSPHERE. 

pidity,  and  floats  off  mostly  toward  the  north,  while  underneath  are 
northerly  winds  which  extend  as  far  as  Greece  and  Italy.  In  North 
Africa,  at  Cairo  and  Alexandria,  there  are  none  but  northerly  winds. 
All  navigators  are  aware  that  in  summer  the  voyage  from  Europe  to 
Africa  is  effected  more  rapidly  than  the  return  passage.  Thus,  if  we 
compare  the  half-duration  of  passages  to  and  fro  between  Toulon  and 
Algiers,  it  will  be  found  that  the  return  passage  is  one-fourth  longer  in 
the  case  of  sailing-vessels,  and  one-tenth  in  the  case  of  steamers.  This 
fact  can  not  be  attributed  to  the  currents,  which  are  very  trifling.  Be- 
sides, the  north  coasts  of  the  islands  of  Majorca  and  Minorca — that  of 
the  latter  in  particular — are  swept  by  this  same  wind,  which  causes  a 
perceptible  stunting  of  vegetation  there.  These  winds  prevail  at  Al- 
giers, Toulon,  and  Marseilles.  In  winter,  on  the  contrary,  when  the 
sand  radiates  considerably,  the  air  of  the  desert  is  colder  than  that  of 
the  sea,  and  in  Egypt  there  is  a  very  cold  south  wind,  though  not  so 
strong  as  the  summer  winds. — Kaemtz  and  Martin. 

To  these  periodical  winds,  to  the  trade-winds  and  the  monsoons,  we 
may  add  the  breezes  caused  upon  sea-coasts  by  the  difference  between 
the  heat  of  the  land  and  of  the  water.  This,  in  the  early  part  of  the 
chapter,  was  pointed  out  as  produced  by  solar  heat,  like  the  trade- 
winds. 

Periodical  and  diurnal  displacement  of  air  takes  place  in  mountain- 
ous regions.  These  consist  in  a  breeze  which  creeps  along  the  side  of 
the  mountain  at  night,  and  in  an  ascending  breeze  during  the  day. 
These  movements  of  air  vary  according  to  the  shape  and  aspect  of  the 
mountains. 

Of  all  the  causes  which  are  assigned  to  the  winds,  one  of  the  most 
powerful  is,  beyond  doubt,  the  condensation  of  vapor  in  the  atmos- 
phere. Sometimes  one  inch  of  water  will  fall,  in  the  course  of  an  hour, 
over  a  wide  tract  of  country,  especially  in  the  equatorial  regions. 
Now,  suppose  this  tract  to  be  Iput  a  hundred  square  leagues  in  extent. 
If  the  vapor  necessary  for  the  production  of  a  depth  of  one  inch  over  a 
hundred  square  leagues  were  in  an  elastic  condition  in  the  air,  and  had 
only  50°  temperature,  it  would  occupy  a  space  a  hundred  thousand 
times  greater  than  in  its  liquid  state ;  that  is  to  say,  it  would  occupy  a 
space  of  a  hundred  square  leagues  by  8860  feet  in  height.  Such,  there- 
fore, would  be  the  dimensions  of  a  void  resulting  from  this  condensa- 
tion. In  reality,  the  vapor  is  not  in  an  elastic  but  in  a  vesicular  state, 
although,  from  the  very  fact  of  its  remaining  suspended  in  the  atmos 


THE  WIND  AND  ITS  CAUSES.  281 

phere,  it  is  probably  of  less  density  than  if  it  were  in  a  liquid  state,  and 
its  condensation  into  drops  of  rain  also  occasions  an  immense  void,  the 
filling  of  which  must  necessarily  give  rise  to  great  atmospheric  disturb- 
ances. 

The  constant  circulation  going  on  in  the  atmosphere  renders  impossi- 
ble the  entire  consumption  of  any  of  the  substances  necessary  to  main- 
tain the  life  of  organized  matter,  such  as  oxygen,  aqueous  vapors,  etc. ; 
and  it  also  prevents  any  dangerous  accumulation  of  deleterious  matter, 
such  as  carbonic  acid.  The  existence  of  animated  nature  is  intimately 
connected  with  this  circulation.  These  simple  features  do  not,  at  first 
sight,  seem  to  apply  to  the  apparently  capricious  play  of  the  weather, 
nor  to  delineate  it  in  its  true  aspect  or  type  of  versatility  and  change- 
ableness.  The  weather  is  not  less  variable,  especially  in  our  climates, 
as  we  shall  presently  see.  We  may  divide  the  surface  of  the  globe 
into  two  unequal  parts — the  regions  of  fixed  and  variable  weather. 
The  state  of  the  air  may  be  predicted  to  the  limit  to  which  the  trade- 
winds  extend,  and  that  for  several  years  to  come.  The  mean  zone  (in- 
cluded between  2°  and  4°  N.  and  S.  latitudes)  is  that  where  throughout 
the  whole  year  great  heat  and  calms  alternate  with  nocturnal  rain-falls 
and  tempests.  Next  to  them,  both  north  and  south,  is  another  zone 
(4°  to  10°  latitude),  where  similar  weather  occurs  only  in  summer  or  in 
winter,  and  trade-winds  render  the  sky  clear.  There  is  a  third  zone 
(10°  to  28°  N.  latitude)  where,  in  winter  as  in  summer,  the  trade-winds 
do  not  usher  in  the  slightest  moisture,  where  years  pass  without  the 
soil  being  refreshed  by  the  least  drop  of  rain. 

Finally,  another  zone,  both  north  and  south  (from  20°  to  30°  latitude), 
which  forms  the  limit  of  fixed  weather ;  there  the  trade-winds  cause  the 
summer  to  be  without  rain  and  the  winter  to  be  mild  and  rainy,  though 
the  rain  is  never  continuous.  The  approximate  indication  of  the  lati- 
tudes refers  to  the  northern  hemisphere  and  the  Atlantic  Ocean,  the 
sole  region  where  reliable  observations  have  been  collected. 

We  now  have  to  consider  a  zone  of  24°  latitude,  where  the  meeting 
between  the  polar  and  the  equatorial  currents  occasions  a  variable  cli- 
mate, which  only  seems  to  us  capricious  and  uncertain  because  the  cir- 
cumstances influencing  the  predominance  of  one  of  the  two  currents 
in  a  given  locality  are  so  complicated  that  we  have  been  unable  to  de- 
duce from  observations  a  law  by  which  these  modifications  can  be  clas- 
sified. If  we  study  the  question  we  find,  as  I  have  said,  that  there  are 
in  reality  but  two  winds  in  the  atmosphere ;  that  which  blows  from  the 


282  THE  ATMOSPHERE. 

poles  toward  the  equator,  and  that  which  makes  its  way  back  from  the 
equator  to  the  poles.  Let  us  now  take  a  place  situated  in  the  region  of 
variable  weather  (the  latitudes  of  Paris,  Vienna,  or  London,  for  instance), 
and  further,  let  us  admit  that  this  place  is  just  in  the  direction  of  the 
polar  current  When  the  north  wind  blows  there,  the  cold  becomes 
accentuated,  the  sky  gets  clear,  even  if  the  wind,  deviating  slightly  from 
its  direction,  turns  toward  the  east.  The  polar  air  which  it  brings  .with 
it  is,  as  Schleiden  remarks,  very  dangerous  for  consumptive  persons,  by 
reason  of  its  extreme  dryness  and  the  abundance  of  oxygen  in  it.  The 
east  wind  blows  until  some  other  wind  comes  to  take  its  place,  and  this 
can  only  be  done  by  the  equatorial  current  which  arrives  as  a  southerly 
wind.  The  immediate  result  produced  by  this  meeting  is  to  give  birth 
to  an  intermediate  direction,  or  to  the  south-east  wind,  the  hot  and  hu- 
mid air  of  which,  cooled  by  the  polar  current,  is  obliged  to  abandon  a 
part  of  its  water  in  the  shape  of  clouds,  snow,  or  rain.  The  equatorial 
current  gradually  gains  the  mastery,  the  weather  clears  up,  becomes 
warmer,  and  maintains  itself  with  a  southerly  wind,  which  imperceptibly 
veers  to  the  west.  There  is  only  the  polar  current  which  can,  in  turn, 
take  its  place ;  the  fusion  of  these,  passing  to  the  north-west,  produces 
abundant  atmospheric  precipitation.  Then  we  have  those  cold  and 
damp  days  which  are  so  unpleasant  to  persons  of  a  nervous  tempera- 
ment. 

Strange  to  say,  this  variable  zone,  which  one  would  be  inclined  to  re- 
gard as  the  most  unfavorable  for  the  development  of  the  human  race, 
embraces  nearly  all  midland  Asia,  Europe,  North  America,  and  the 
north  coast  of  Africa,  and  consequently  comprises  the  scene  upon  which 
has  been  illustrated  the  history  of  humanity  and  of  its  intellectual  de- 
velopment. Perhaps  there  is  some  secret  connection  between  this  phe- 
nomenon and  the  special  development  of  the  vegetable  world  in  this  re- 
gion. 

This  sketch  of  the  distribution  of  weather  over  the  surface  of  the 
globe  is  modified  by  many  causes.  The  elevation  of  countries  above 
the  sea-level,  plains  and  mountains,  sandy  deserts  and  forests,  cause 
great  disturbances  in  the  action  of  these  laws. 

Among  the  influences  which  modify  weather,  one  of  the  most  impor- 
tant is  the  manner  in  which  the  sea  and  the  land  are  spread  over  the 
surface  of  the  globe.  The  land,  being  exposed  to  the  solar  rays,  is  heat- 
ed more  rapidly  than  the  sea,  and,  after  a  certain  interval,  attains  a 
higher  temperature,  which,  moreover,  cools  again  far  more  slowly.  The 


THE  WIND  AND  ITS  CAUSES.  283 

first  consequence  is  that  the  hottest  zone,  the  region  of  calms,  is  not 
equally  extensive  both  to  the  north  and  to  the  south  of  the  equator ; 
but,  on  the  contrary,  occupies  the  largest  space  in  the  northern  hemi- 
sphere. 

We  have  seen  that  heat  and  its  unequal  distribution  in  all  direc- 
tions is  the  fundamental  phenomenon  around  which  the  others,  which 
are  dependent  upon  it,  group  themselves.  The  moisture  of  the  air  has 
an  intimate  co-relation  with  this  phenomenon,  and  the  latter,  together 
with  the  heat,  are  the  causes  of  vegetable  life.  It  is  upon  these  two 
conditions  that  principally  depends  the  distribution  of  plants  over  the 
globe.  The  animal  world  follows  the  plants,  for  the  existence  of  herbiv- 
orous beings  is  directly  connected  with  that  of  the  carnivora.  The  first 
supreme  principle,  that  which  not  only  vivifies,  but  stirs  up  and  regu- 
lates all,  is  the  sun ;  its  rays  are  the  pencils  with  which  it  traces  light 
and  shadow,  the  burning  yellow  of  the  arid  sand,  and  the  fresh  green 
of  meadows,  and  even  the  sketch  of  an  ethnographical  map  for  the  hu- 
man race. 


284  THE  ATMOSPHERE. 


CHAPTER  II. 
THE  SEA  CURRENTS:  METEOROLOGY  or  THE  OCEAN— MARITIME  ROUTES 

— THE   GULF  STREAM. 

WE  have  seen  that  the  distribution  of  solar  heat  over  the  globe  cre- 
ates in  the  atmosphere  a  general  regular  circulation.  In  the  next  chap- 
ter we  will  prove  that  the  irregular  and  variable  winds  are  alike  due  to 
this  heat,  and  that  they  are  subject  to  laws  of  periodicity  which  science 
is  engaged  in  studying.  But,  before  having  done  with  the  great  cur- 
rents of  the  atmosphere,  it  is  necessary  that  we  should  form  some  idea 
of  the  great  ocean  currents,  also  dependent  on  the  action  of  the  very 
same  heat  which  regulates  all  things  here  below. 

The  sea  is  not  motionless;  neither  its  waters  nor  the  atmosphere 
above  them.  A  great  general  oscillation  of  the  surface  occurs  twice  a 
day,  under  the  attracting  influences  of  the  moon  and  sun  :  these  oscilla- 
tions are  the  tides,  whose  flux  and  reflux  alternately  cover  and  lay  bare 
the  shores  of  the  ocean,  and  give  to  the  coast  that  endless  variety  which 
never  fails  to  charm  us.  This  movement  of  the  waters  is  due  to  an  as- 
tronomical cause,  and  need  not  be  gone  into  here.  But  the  sea  is  ani- 
mated by  another  meteorological  circulation,  more  complex  and  wider, 
which  may  almost  be  compared  to  the  circulation  of  the  blood  in  our 
veins ;  it  is  traversed  by  currents  which,  running  from  the  equator  to 
the  poles,  and  vice  versa,  thus  forming  a  connecting  link  between  the 
most  distant  seas,  distribute  heat  to  colder  regions,  exercjse  a  cooling 
influence  within  the  torrid  zones,  equalize  the  briny  and  chemical  com- 
position of  the  ocean,  and  form,  to  a  certain  extent,  the  vital  circuit  of 
the  globe ;  like  the  sap  which  rises  and  falls  in  plants,  like  the  blood 
which  becomes  regenerated  at  the  heart  after  having  carried  its  tribute 
to  the  farthest  extremities  of  the  organization.  • 

These  ocean  currents  merit  our  special  attention,  and  their  study  will 
embrace  at  once  the  currents  of  the  atmosphere  which  accompany  and 
complete  them,  constituting  the  meteorology  of  the  ocean.  Both  have, 
especially  for  the  last  thirty  years,  been  the  subject  of  detailed  research. 

Maritime  travel  differs  db  initio  from  journeys  by  land,  in  the  absence 
of  any  fixed  route.  For  a  long  period,  indeed,  modern  navigators  nev- 


THE  SEA  CURRENTS.  285 

er  suspected  that  there  existed  upon  the  surface  of  the  ocean  numerous 
highways,  traced  by  the  hand  of  nature.  The  constancy  of  the  mon- 
soons, the  periodical  return  of  the  marine  breezes  along  the  coasts  of 
the  Red  Sea  and  in  the  Indian  Ocean,  are  phenomena  which  our  fore- 
fathers had  ascertained  and  utilized.  When  the  astronomer  Hippalus 
discovered  the  physical  fact  of  the  return  journey  of  the  summer  mon- 
soon, he  made  a  discovery  which  the  Arabian  sailors  had  for  centuries 
been  acquainted  with,  and  which  they  had  taken  advantage  of  to  pre- 
serve the  monopoly  in  the  trade  of  Ceylon  spices  and  perfumes,  which 
they  sold  as  the  products  of  Arabia.  The  discovery  of  Hippalus  caused 
a  complete  revolution  in  the  system  of  maritime  services  among  the 
Europeans  who  flourished  at  the  commencement  of  the  Christian  era. 
The  discoveries  effected  by  the  researches  of  Lieutenant  Maury,  of  the 
Washington  Observatory,  during  our  own  day,  are  analogous  to  the 
above,  but  on  a  much  larger  scale.  On  account  of  their  great  inter- 
course with  other  peoples,  and  the  geographical  position  of  their  coun- 
try, which  is  bounded  by  two  oceans,  the  Americans  were  more  inter- 
ested than  any  other  nation  in  the  discovery  of  the  shortest  sea-routes. 
To  effect  this  purpose,  it  was  necessary  to  compare  with  each  other  the 
thousands  of  routes  that  had  been  followed  by  thousands  of  navigators. 
This  herculean  task  rendered  it  possible  to  deal  with  the  whole  globe 
as  Hippalus  had  dealt  with  the  short  distance  between  Egypt  and  Ta- 
probane. 

The  great  navigators  of  early  ages  seemed  to  have  struck  out  the 
only  routes  practicable,  without  its  occurring  to  them  to  introduce  the 
modifications  which  the  comparative  study  of  the  data  of  experience 
might  have  led  them  to.  But  when  the  application  of  steam  to  the 
means  of  transport  had  proved  the  advantages  of  a  rapid  system  of  in- 
tercommunication, and  the  great  value  of  time,  attention  naturally  be- 
came turned  to  the  discussion  of  better  routes,  and  the  means  of  decid- 
ing as  to  how  they  could  be  arrived  at.  A  steam  vessel,  taking  no  ac- 
count of  the  wind,  can  trace  upon  the  sphere  the  shortest  and  the  most 
direct  line  between  its  point  of  departure  and  its  place  of  arrival;  but 
with  the  sailing  vessel,  subject  as  it  is  to  aerial  currents  which  consti- 
tute its  sole  means  of  progression,  the  line  which  is  shortest  in  point  of 
distance  often  becomes  the  longest  in  respect  to  the  time  occupied  in 
traveling  along  it.  To  find  the  greatest  possible  sum  of  favorable  winds, 
without  deviating  more  than  can  be  avoided  from  the  straight  line,  is 
the  surest  way  to  accomplish  a  quick  passage.  The  observations  taken 


286  THE  ATMOSPHERE. 

at  the  surface  of  the  seas  by  navigators  were  long  allowed  to  remain 
profitless  for  the  purposes  of  science  and  navigation.  Under  Maury's 
auspices  they  led,  in  a  few  years,  to  a  knowledge  of  the  general  circula- 
tion of  the  atmosphere  and  the  seas.  At  the  same  time  they  have  been 
instrumental  in  reducing  by  a  fourth,  a  third,  and  even  a  half,  in  some 
instances,  the  length  of  long  voyages,  and  in  effecting  an  immense  sav- 
ing in  the  cost  of  transport. 

To  awaken  public  interest  by  some  practical  result  which  would  dem- 
onstrate the  great  importance  of  these  new  studies,  he  concentrated  all 
his  efforts  upon  one  single  route — that  from  the  United  States  to  Rio 
Janeiro.  The  data  which  he  collected  enabled  him  to  ascertain  a  route 
far  shorter  and  better  than  that  followed  by  the  great  mass  of  naviga- 
tors. The  ship  Wright,  Captain  Jackson,  from  Baltimore,  was  the  first 
to  steer  by  Maury's  course.  Starting  from  Baltimore,  on  the  9th  of 
February,  1848,  this  vessel  crossed  the  equator  in  twenty -four  days, 
while  the  time  occupied  had  previously  averaged  forty-one  days. 

This  route  from  the  United  States  to  the  equator  is  all  the  more  im- 
portant because  it  is  the  road  of  all  ships  sailing  from  North  America  to 
the  southern  hemisphere,  whether  their  ultimate  destination  be  the  Pa- 
cific, the  Indian  Ocean,  or  the  Atlantic.  From  forty-one  days  this  pas- 
sage was  reduced  to  twenty-four  days,  afterward  to  twenty,  and  finally 
as  low  as  eighteen.  This  is  a  gain  of  fifty  per  cent. 

The  passage  from  the  States  to  California  took,  on  an  average,  rather 
more  than  180  days ;  after  Maury  had  brought  his  knowledge  to  bear 
upon  the  subject,  it  was  at  first  shortened  to  135  days,  and  since  then  to 
100 ;  while  one  of  the  fleet  of  clippers  trading  there — the  Flying  Fish — 
cast  anchor  in  the  harbor  of  San  Francisco  on  the  92d  day  after  leaving 
New  York. 

But  the  most  remarkable  instance  is  furnished  by  the  voyage  to  Aus- 
tralia. From  England  to  Sydney,  a  vessel  sailing  under  the  old  sys- 
tem used  to  take  at  least  125  days,  which  was  the  usual  average.  The 
return  journey  being  about  the  same,  the  total  length  of  the  voyage 
amounted  to  250  days.  When  Maury  passed  through  England  to  at- 
tend the  Congress  at  Brussels,  he  promised  the  British  sailors  and  mer- 
chants that,  as  a  recognition  of  the  help  they  had  afforded  him,  he 
would  diminish  by  at  least  a  month  the  voyage  to  Australia,  and  reduce 
the  return  passage  to  a  still  greater  extent;  or,  in  other  words,  lessen 
by  a  quarter  the  distance  between  England  and  its  wealthy  colony.  A 
little  later,  when  the  notions  with  respect  to  this  route  were  complete, 


THE  SEA  CURRENTS.  287 

Maury  pointed  out  the  immense  advantage  that  would  be  derived  by 
making  the  voyage  to  Australia  a  regular  circumnavigation  of  the 
globe — that  is,  doubling  the  Cape  of  Good  Hope  on  the  outward  voy- 
age, and  Cape  Horn  on  the  return  passage.  The  total  length  of  these 
two  voyages  would,  he  said,  occupy  130  days,  or  even  less,  in  place 
of  250,  as  were  taken  before.  This  prediction  has  been  fulfilled, 
and  even  exceeded,  the  saving  of  time  being  equivalent  to  fifty  per 
cent. 

Let  us  see  what  is  the  economy  from  a  pecuniary  point  of  view. 
The  price  of  freight  to  Australia  is  about  one  shilling  per  ton  per  day. 
Taking  the  average  tonnage  of  the  vessels  upon  this  line  as  being  only 
500  (they  are  700  in  reality),  and  assuming  a  reduction  of  only  thirty 
days  in  the  passage,  it  will  result  that  each  ship  will  have  realized  in 
each  passage  a  saving  of  15,000  shillings.  If  we  take  Maury's  calcula- 
tions, and  put  the  number  of  vessels  of  all  flags  that  ply  annually  be- 
tween the  North  Atlantic  ports  and 'Australia  at  1800,  there  will  be  a 
clear  gain  of  twenty-five  millions  of  shillings  at  the  expiration  of  a 
twelvemonth. 

For  English  commerce  alone,  in  the  Indian  Ocean,  the  annual  econo- 
my is  nearly  £500,000.  Taking  all  passages  effected  by  ships  of  vari- 
ous nations,  this  discovery  must  effect  an  annual  saving  of  four  millions 
of  pounds  sterling. 

The  greater  the  distance  to  be  accomplished,  the  greater  is  the  advan- 
tage in  deviating  from  the  straight  line  to  seek  a  region  where  continu- 
ous breezes  will  impel  the  vessel  at  the  greatest  speed.  Thus,  generally 
speaking,  if  one  is  sailing  from  east  to  west,  it  is  in  the  intertropical  re- 
gion that  the  speed  is  greatest;  whereas,  in  order  to  sail  very  rapidly 
from  west  to  east,  it  is  necessary  to  go  beyond  the  tropics,  either  north 
or  south. 

Each  day's  delay  in  the  arrival  of  a  merchant- vessel  beyond  the  fixed 
date  or  the  average  of  passages  is  not  only  a  more  or  less  considerable 
cause  of  annoyance  to  the  passengers,  whose  health,  and  even  life,  may 
be  depending  upon  their  speedy  arrival ;  it  is  also  a  cause  of  loss  to  the 
shipper  and  the  merchant.  The  expenses  of  a  large  vessel  vary,  as  Ad- 
miral Fitzroy  pointed  out — including  wages,  provisions,  material,  a  full 
cargo,  and  an  average  number  of  passengers — from  £50  to  £200  a  day ; 
moreover,  to  these  immediate  expenses  must  be  added  the  diminution 
in  the  annual  earnings  of  the  vessel  which  are  consequent  upon  the 
forced  delay  in  its  next  departure.  The  evils  incident  upon  a  long  pas- 


288  THE  ATMOSPHERE. 

sage  are,  therefore,  complex  in  nature,  affecting  the  interest  of  the  ship- 
per and  of  the  public  at  large. 

The  progress  realized  by  the  "Sailing  Directions  "  in  shipping  indus- 
try is,  consequently,  equivalent  to  that  effected  by  the  adjunction  of  a 
new  motive  power.  Thus  a  ship  which,  sailing  in  the  ancient  track, 
would  have  been  at  sea  for  one  hundred  days,  now  follows  the  new 
course,  and  reaches  its  destination  in  half  the  time,  and  is  thus,  so  to 
speak,  supplied  with  a  traction-engine  powerful  enough  to  double  its 
speed.  These  fortunate  results  have  been  universally  accepted.  In  a 
conference  held  at  Brussels  in  1853,  the  United  States,  France,  England, 
Kussia,  Sweden,  Norway,  Denmark,  Holland,  Belgium,  and  Portugal 
agreed  upon  a  uniform  plan  of  meteorological  observations  at  sea,  and 
this  plan  was  soon  adopted  by  Prussia,  Austria,  Spain,  Italy,  and  Bra- 
zil. Since  then,  all  the  trans-oceanic  vessels  belonging  to  these  powers 
have  become  floating  observatories,  which  register  night  and  day  all  the 
incidents  of  navigation  calculated  to  secure  a  complete  knowledge  of  the 
movements  of  the  atmosphere  and  the  sea. 

Thanks  to  these  researches  and  to  the  development  in  late  years  of 
meteorological  observations,  I  am  enabled  to  give,  in  the  previous  and 
following  chapter,  a  general  sketch  of  the  distribution  of  winds  over  the 
surface  of  tlie  globe. 

Let  us  now  consider  the  circulation  of  water,  also  due  to  the  influence 
of  solar  heat. 

It  is  well  known  that  the  seas  are  divided,  first  into  three  great  oceans, 
viz.,  the  Atlantic,  which  separates  Europe  and  Africa  from  America; 
the  Pacific,  which  covers  half  of  the  globe  between  the  two  Americas 
upon  the  one  hand,  and  upon  the  other  Eastern  Asia  and  New  Holland, 
with  the  Archipelago  between ;  and  thirdly,  the  small  ocean  known  as 
the  Indian  Ocean,  which  is  almost  entirely  upon  the  south  of  the  equa- 
tor between  Africa,  Asia,  and  New  Holland. 

If  the  two  great  oceans  be  divided  into  two  parts;  that  to  the  north 
and  that  to  the  south  of  the  equator,  and  if  the  polar  seas  be  taken  into 
account,  we  shall  have  altogether  seven  divisions  in  which  the  move- 
ment of  the  hot  or  cold  waters,  their  flow  from  the  equator  toward  the 
poles,  and  their  return  to  the  point  from  which  they  started,  can  be1 
studied.  It  is  to  this  movement  that  are  due,  throughout  the  sea,  cur- 
rents of  hot  and  currents  of  cold  water,  the  majestic  and  steady  changes 
of  which,  and  the  more  or  less  varying  temperature  of  which,  give  rise 
to  effects  of  a  far  more  important  nature  in  the  economy  of  climates 


THE  SEA  CURRENTS.  289 

than  might  be  supposed  by  those  whose  only  knowledge  of  the  globe  is 
derived  from  ordinary  maps. 

Let  us  analyze  and  weigh  these  important  currents,  taking  as  an  ex- 
ample the  circuit  formed  by  the  waters  of  the  Atlantic  to  the  north  as 
being  best  known  to  us,  and  which  is  continually  being  traversed 
by  vessels  coming  and  going  between  Europe  and  North  or  Central 
America. 

In  the  equatorial  regions,  the  waters  of  the  ocean  are  impelled  to- 
ward the  west  by  an  incessant  movement  which,  in  the  Atlantic,  car- 
ries them  toward  tropical  America.  This  vast  current,  30°  in  width, 
twenty  of  which  are  to  the  north  and  ten  to  the  south,  breaks  against 
the  shores  of  the  New  World.  In  accordance  with  the  shape  of  Amer- 
ica, the  eastern  point  of  which  is  a  long  way  below  the  equator,  the 
greater  part  of  these  waters  make  their  way  toward  the  Gulf  of  Mexico, 
the  bends  of  which  it  follows,  and  finally  makes  its  way  out  again  by 
the  extreme  point  of  Florida,  running  along  the  coast  of  the  United 
States  from  south  to  north.  This  gulf,  situated  in  the  torrid  zone,  is  or» 
all  sides  surrounded  by  lofty  mountains,  which  shut  in  the  solar  rays  as 
within  a  vast  funnel,  and  store  up  therein  the  heat  of  a  burning  climate. 
It  is  from  this  focus  that  the  equatorial  current  starts.  It  runs  across 
the  Straits  of  Florida,  and  produces  an  impetuous  stream,  nearly  1000 
feet  deep  and  fourteen  leagues  wide,  running  at  the  speed  of  five  miles 
an  hour.  Its  waters,  which  are  warm  and  very  salt,  are  of  the  color  of 
indigo  blue,  and  differ  from  their  greenish  borders  formed  by  the  waves 
of  the  sea.  This  mass  creates  in  its  passage  a  great  agitation,  and  thus 
follows  its  course  without  becoming  confounded  with  the  ocean.  Shut 
in  between  two  liquid  walls,  the  waters  of  the  Gulf  Stream  form  a  mov- 
ing vault  which  glides  along  the  sea,  carrying  off  to  a  great  distance  all 
objects  which  get  drifted  into  it.  "In  the  greatest  droughts  it  never 
fails,  in  the  greatest  floods  it  never  runs  over.  Nowhere  in  the  world 
does  there  exist  so  majestic  a  current.  It  is  more  rapid  than  the  Ama- 
zon, more  impetuous  than  the  Mississippi ;  and  the  collective  waters  of 
these  two  streams  would  not  equal  the  thousandth  fraction  of  the  vol- 
ume of  water  which  it  displaces." — Maury. 

By  means  of  the  thermometer  the  navigator  can  follow  the  great 
liquid  vein.  The  instrument,  plunged  alternately  into  its  edges  and  its 
mid-stream,  shows  a  difference  of  27°  of  temperature. 

Powerful  and  rapid  the  Gulf  Stream  runs  northward,  following  the 
coast  of  tne  United  States,  as  far  as  the  Banks  of  Newfoundland.  There 

19 


290  THE  ATMOSPHERE. 

it  encounters  the  tremendous  shock  of  a  polar  current,  upon  which  are 
floating  enormous  icebergs,  veritable  mountains  of  ice,  the  force  of 
which  is  such  that  one  of  them,  weighing  more  than  twenty  billions  of 
tons,  carried  the  vessel  commanded  by  Lieutenant  de  Haven  more  than 
three  hundred  leagues  southward.  The  Gulf  Stream,  whose  waters  are 
lukewarm,  dissolves  the  floating  ice.  The  icebergs  melt,  and  the  earth, 
and  even  the  fragments  of  rock  which  they  contained,  are  swallowed  up 
by  the  waters. 

Upon  reaching  the  neighborhood  of  Europe,  it  sends  a  great  part  of 
its  waters  in  the  direction  of  the  Polar  Sea,  along  the  coasts  of  Ireland, 
Scotland,  and  Norway;  'the  remainder  turns  off  to  the  south,  opposite 
the  west  coast  of  Spain,  and  regains  the  great  tropical  current  off  the 
centre  of  Africa.  After  having  effected  their  junction  .with  this  cur- 
rent, of  which  they  are,  so  to  speak,  the  source,  its  waters  make  their 
way  westward,  to  reach  once  more  the  coasts  of  Mexico  and  the  United 
States,  and  to  traverse,  for  the  second  time,  the  space  which  separates 
the  United  States  from  Europe,  thus  forming  a  continuous  circuit  from 
Africa  to  Mexico,  returning  to  the  point  from  which  they  started  by  the 
route  given.  The  bottles  which  sailors  throw  into  the  sea,  with  a  men- 
tion of  the  day  and  the  spot  where  they  were  confided  to  the  ocean, 
have  shown  us  that  this  voyage  of  from  13,000  to  19,000  miles  is  ac- 
complished in  three  years  and  a  half.  The  winds  have  about  the  same 
direction  as  the  waters,  that  is  to  say,  that  between  the  tropics  the  east- 
erly trade-winds  prevail,  driving  the  atmosphere  from  Africa  to  Ameri- 
ca, just  as  the  tropical  current  conveys  the  water  thither.  Between  the 
United  States  and  Europe,  just  as  this  current  causes  the  sea  to  flow 
eastward,  so  also  do  the  counter-currents  of  the  trade-winds  blow  toward 
Europe,  whence  it  happens  that  the  passage  from  the  United  States  to 
Europe  is  effected  more  rapidly  than  the  return  journey,  for  in  this  lat- 
ter case  the  wind  and  the  current  are  against  the  vessel.  It  is  well 
known  that  when  Christopher  Columbus  ventured  to  give  himself  up  to 
the  west  winds,  he  got  as  low  down  as  Africa  to  take  advantage  of  the 
easterly  winds  which,  according  to  his  calculation,  would  lead  him  to 
China.  As  the  late  M.  Babinet  remarks,  it  is  difficult  to  understand 
how,  at  this  epoch,  when  geographical  knowledge  was  sufficiently  ad- 
vanced to  permit  of  the  globe's  dimensions  and  the  distance  from  India 
and  China  being  pretty  accurately  known,  any  one  could  have  expected 
to  reach  the  eastern  coasts  of  China  after  a  navigation  equal  to  three 
or  four  times  the  distance  beween  the  Old  and  the  New  World.  If 


THE  SEA  CURRENTS.  291 

America  had  not  been  in  existence,  he  would  have  perished  a  hundred 
times  over  before  he  could  have  reached  China. 

Before  passing  to  the  other  maritime  circuits  analogous  to  these  of 
the  North  Atlantic,  let  us  consider  carefully  the  circumstances  by  which 
it  is  characterized. 

The  tropical  waters,  in  their  journey  from  the  coasts  of  Africa  to 
those  of  America,  pass  beneath  the  rays  of  a  zenithal  sun,  and  are  con- 
tinually being  heated  until  they  reach  the  Gulf  of  Mexico ;  they  then 
flow  by  way  of  the  Straits  of  Bahama,  where  they  form  a  rapid  current 
of  hot  water,  which  re-ascends  to  the  east  of  the  United  States,  toward 
the  Banks  of  Newfoundland.  There  the  current  turns  eastward  on  its 
way  to  Europe,  but  still  preserves  the  high  temperature  due  to  its  trop- 
ical origin,  and  this  is  one  of  the  most  powerful  agencies  of  nature  for 
increasing  the  temperature  of  our  globe — viz.,  the  conveyance,  by  means 
of  these  waters,  of  the  heat  which  the  sun  sheds  between  the  tropics  to- 
ward the  northern  regions.  In  proportion  as  this  current  advances,  it 
parts  its  heat,  which  it  distributes  into  the  atmosphere  and  over  the 
seas  which  it  traverses ;  then  it  returns,  leaving  Spain  and  the  north  of 
Africa  to  its  left,  to  resume  its  place  in  the  tropical  current,  and  again 
to  receive  heat,  which  it  will  transfer  as  before  to  European  latitudes. 

It  is  by  means  of  the  winds  that  the  heat  of  the  sea  communicates  it- 
self to  the  main-land.  We  shall  see  presently  that  in  Europe  the  pre- 
vailing winds  of  the  globe' are  westerly,  inclining  to  south-west.  It  is 
seen  at  once  that  these  currents  of  air,  having  a  current  of  hot  water  for 
basis,  will  share  its  temperature,  and  pass  over  Europe  and  be  much 
warmer  than  if  the  sea,  deprived  of  this  warm  current,  had  only  the 
same  degree  of  warmth  as  is  due  to  latitude.  To  demonstrate  this  as- 
sertion, we  have  only  to  compare  the  climates  and  temperatures  of 
American  cities  with  those  of  France  and  England  which  are  in  the 
same  latitudes. 

None  of  the  masses  of  water  which  move  from  place  to  place  in  the 
seas  merit  such  close  attention  as  that  of  the  Gulf  Stream ;  none  are  of 
greater  importance  in  regard  to  the  commerce  of  nations,  nor  exercise  a 
greater  influence  upon  climate.  It  is  to  the  Gulf  Stream  that  the  Bri- 
tannic isles,  France,  and  neighboring  countries,  owe,  in  a  great  measure, 
their  mild  temperature,  their  agricultural  wealth,  and,  moreover,  a  very 
large  part  of  their  material  and  moral  strength.  Its  history  is  almost 
that  of  the  whole  of  the  North  Atlantic,  so  great  is  the  hydrological  and 
climacteric  influence  of  this  current  of  the  seas. 


292  THE  ATMOSPHERE. 

Owing  to  the  rotatory  motion  of  the  globe,  and  probably  also  to  the 
general  direction  of  the  coasts,  the  current  follows  without  intermission 
a  north-easterly  course,  and  comes  in  contact  with  none  of  the  advanced 
points  of  the  continent.  Beyond  New  York  and  Cape  Cod  it  bends  far- 
ther eastward,  and,  ceasing  to  run  parallel  with  the  American  coast, 
turns  off  into  the  mid- Atlantic,  toward  the  shores  of  Western  Europe. 
As  Maury  says:  "If  an  enormous  cannon  could  fire  a  ball  from  the 
Strait  of  Bahama  to  the  North  Pole,  the  projectile  would  follow  almost 
exactly  in  the  curve  or  course  of  the  Gulf  Stream,  and,  deviating  grad- 
ually as  it  went,  would  reach  Europe  traveling  eastward." 

From  the  43d  to  the  47th  degree  of  north  latitude,  in  the  neighbor- 
hood of  the  Banks  of  Newfoundland,  the  Gulf  Stream,  traveling  from 
the  south-west,  encounters  upon  the  surface  of  the  sea  the  polar  current. 
The  line  of  demarkation  between  these  oceanic  streams  is  never  abso- 
lutely the  same,  and  varies  with  the  seasons.  In  winter — that  is,  from 
September  to  March — the  cold  current  drives  the  Gulf  Stream  toward 
the  south ;  for,  during  this  season,  all  the  circulatory  system  of  the  At- 
lantic—  winds,  rain,  and  currents — veer  toward  the  southern  hemi- 
sphere, above  which  the  sun  is  situated.  In  summer — that  is,  from 
March  to  September — the  Gulf  Stream  regains  the  preponderance,  and 
repels  the  polar  current  farther  north.  After  having  come  in  collision 
with  the  waters  of  the  Gulf  Stream,  those  of  the  arctic  current  cease,  in 
a  great  measure,  to  flow  upon  the  surface,  a"nd  sink  by  reason  of  their 
being  cold,  and  consequently  heavy.  It  is  easy  to  trace  the  direction 
of  this  counter-current,  which  is  exactly  opposite  to  that  of  the  Gulf 
Stream,  by  the  mountains  of  ice  which  the  mild  temperature  of  lower 
latitudes  fails  to  melt,  and  which  float  in  a  south-easterly  direction,  un- 
til they  meet  the  superficial  current,  which  they  cleave  like  the  prow  of 
a  vessel.  Farther  south,  it  is  only  by  sounding  that  the  existence  can 
be  ascertained  of  this  hidden  current,  the  cold  waters  of  which  serve 
as  a  bed  to  the  warm  stream  proceeding  from  the  Gulf  of  Mexico.  It 
descends  lower  and  lower  until  it  reaches  the  Straits  of  the  Bahama 
Islands,  where  the  thermometer  indicates  it  at  a  depth  of  1300  feet. — 
Eedus. 

We  have  the  pendant  of  the  Gulf  Stream  in  the  Pacific  Ocean,  in  the 
shape  of  a  warm  current  which  follows  the  coasts  of  China  and  Japan, 
and  which  has  long  been  known  to  Japanese  geographers  by  the  name 
of  Kuro-Siwo  (the  Black  Stream) — a  name  which  originated,  no  doubt, 
in  the  dark  hue  of  its  waters.  In  the  southern  seas  the  currents  are 


THE  SEA  CURRENTS.  293 

not  so  well  known  to  us,  and  are,  in  fact,  much  less  numerous.  It  is, 
moreover,  probable  that  the  marine  streams  are  not  isolated  currents, 
but  several  portions  of  one  net-work,  distinct  veins  in  a  comprehensive 
system  of  circulation. 

The  quantity  of  heat  which  the  Gulf  current  carries  northward  forms 
a  very  considerable  part  of  the  caloric  which  is  stored  up  in  the  waters 
of  the  torrid  zone.  The  total  heat  of  the  current  would  suffice,  if  it 
were  concentrated  upon  a  single  point,  to  melt  mountains  of  iron  and  to 
form  a  stream  of  metal  as  voluminous  as  the  Mississippi ;  it  would,  fur- 
ther, suffice  to  raise  from  winter  to  summer  temperature  the  whole  col- 
umn of  air  which  lies  over  France  and  Great  Britain. 

Notwithstanding  the  march  of  the  sun,  it  is,  upon  an  average,  as 
warm  in  Ireland  at  52°  N.  latitude  as  it  is  in  the  United  States  at  38° 
N.  latitude,  or  a  place  more  than  1000  miles  nearer  the  equator. 

The  Gulf  current,  which  carries  the  tropical  heat  to  the  temperate  re- 
gions of  Europe,  often  serves,  too,  as  a  highway  for  the  hurricanes; 
hence  the  names  of  Weather-breeder  and  Storm-king,  which  have  been 
given  to  the  Gulf  Stream.  The  movements  of  the  atmospheric  ocean 
and  those  of  the  ocean  of  waters  are  so  completely  parallel  that  we  are 
tempted  to  view  them  as  one  and  the  same  phenomenon  both  in  the 
currents  aerial  and  marine.  Thus  the  Gulf  Stream  seems  to  be  for  the 
winds  what  it  in  reality  is  for  the  waters — the  great  intermediary  be- 
tween the  two  worlds.  It  -transmits  to  the  seas  of  Northern  Europe  the 
saline  matters  of  the  Gulf  of  the  Antilles;  it  carries  with  it  the  tropical 
heat  for  the  benefit  of  the  temperate  regions ;  it  marks  the  route  follow- 
ed by  the  torrents  of  electricity  proceeding  from  the  storms  in  the  An- 
tilles. It  is,  in  fact,  the  great  serpent  of  the  Scandinavian  poets,  which 
displays  its  immense  ring  along  the  ocean,  and  which,  by  the  motion  of 
its  head,  either  causes  a  mild  breeze  to  blow  or  emits  the  raging  hurri- 
cane. While,  in  the  North  Atlantic,  the  equatorial  current,  which  falls 
into  the  Gulf  of  Mexico,  returns  from  whence  it  came,  traversing  high 
latitudes,  another  part  of  this  current,  much  less  voluminous,  after  hav- 
ing touched  Cape  St.  Koch,  which  forms  the  eastern  extremity  of  South- 
ern America,  descends  along  the  eastern  coast  of  that  same  continent, 
and  then,  crossing  the  Atlantic  from  west  to  east,  returns  toward  Lower 
Africa,  running  along  its  western  shores  and  rejoining,  by  the  south, 
the  great  tropical  current,  just  as  the  Gulf  Stream  meets  it  northward. 
Down  to  the  quantity  even  of  water  which  it  contains,  this  current 
bears  a  marked  resemblance  to  the  circuit  which  occupies  the  north  of 


294  THE  ATMOSPHERE. 

this  ocean.  The  portion  which  runs  off  beyond  the  tropics,  and  which 
returns  from  west  to  east,  from  South  America  to  South  Africa,  is  also 
a  current  of  hot  water,  like  the  Gulf  Stream  between  the  United  States 
and  Europe.  The  comparison  of  the  masses  of  water  which  each  of 
these  circuits  separately  conveys  shows  how  much  better  the  north  is 
provided  with  hot  waters  than  the  'south.  It  is  not  too  much  to  say 
that  the  north  circuit  forms  a  current  five  or  six  times  more  abundant 
than  the  south  circuit.  If  we  now  consider  the  Pacific  Ocean,  there 
also  we  find  tropical  waters  which  flow  on  to  the  shores  of  New  Hol- 
land, the  Northern  Archipelago,  and  Lower  Asia.  Most  of  these  waters 
re-ascend  northward  in  vast  currents  of  lukewarm  water  which  give 
to  High  California  and  to  Oregon  climates  very  similar  to  those  of 
Europe. 

The  North  and  South  Atlantic,  the  North  and  South  Pacific,  and  the 
Indian  Ocean,  each  contain  a  current,  that  of  the  former  ocean  being  the 
most  voluminous.  The  Arctic  seas,  north  and  south,  also  appear  to  be 
traversed  by  a  current  running  eastward,  round  the  Pole. — Babinet. 

The  circulation  of  the  sea  is  completed  by  submarine  currents.  There 
must  exist  one  of  these,  conveying  the  waters  of  the  Mediterranean  into 
the  Atlantic.  Its  existence  is,  in  a  way,  demonstrated  by  a  calculation 
which  shows  that  the  quantity  of  salt  water  in  the  upper  current  of  the 
Straits  of  Gibraltar  is  2900  cubic  miles  per  year,  the  quantity  of  soft 
water  contributed  by  the  rivers  240  cubic  miles,  and  that  which  is  lost 
by  evaporation  480  cubic  miles.  Thus  there  would  be  an  annual  ex- 
cess of  2660  cubic  miles,  if  the  equilibrium  were  not  re-established  by 
a  submarine  current.  This  hypothesis " seems  to  have  received  confir- 
mation by  a  very  curious  fact. 

Toward  the  close  of  the  seventeenth  century  a  Dutch  brig,  pursued 
by  a  French  corsair,  the  Phoenix,  was  overtaken  between  Tangier  and 
Tarifa,  and  disabled  by  a  single  cannonade.  Instead  of  sinking  at  once, 
the  brig,  which  had  a  cargo  of  oil  and  alcohol,  floated  beneath  the  sur- 
face of  the  waters,  and  did  not  finally  go  to  the  bottom  for  two  or  three 
days,  after  having  been  carried  twelve  miles  toward  Tangier  from  the 
point  at  which  she  first  disappeared.  It  was  evidently  carried  this  dis- 
tance by  an  under-current,  in  an  opposite  direction  to  that  of  the  sur- 
face-current. This  fact,  in  conjunction  with  some  recent  experiments, 
confirms  the  opinion  which  admits  the  existence  of  a  current  issuing 
from  the  Straits  of  Gibraltar.  Lieutenant  Maury  also  considered  it 
certain  that  there  is  a  submarine  counter-current  to  the  south  of  Cape 


THE  SEA  CURRENTS.  295 

Horn,  which  carries  the  overflow  of  the  Atlantic  into  the  Pacific.  As 
a  matter  of  fact,  the  Atlantic  is  continually  being  fed  by  very  large 
rivers ;  whereas  the  Pacific,  into  which  debouches  no  important  stream, 
must,  on  the  other  hand,  lose  an  immense  body  of  water,  owing  to  the 
evaporation  which  takes  place  from  its  surface. 

Certain  lower  currents  have  been  ascertained  by  weighting  a  piece 
of  wood  and  plunging  it  into  the  water,  keeping  hold  of  it  at  the  same 
time  with  a  piece  of  string,  so  as  to  let  it  sink  to  any  depth  which  may 
be  desired.  At  the  other  end  of  the  line  is  attached  an  empty  barrel 
strong  enough  to  support  the  apparatus,  and  then  the  whole  is  set  free. 
The  sailors  who  tried  this  experiment  for  the  first  time  were  astonished 
to  find  this  little  barrel  traveling  in  an  opposite  direction  to  the  wind 
and  the  sea  at  the  rate  of  a  knot  or  more  per  hour.  The  crew  were 
even  inclined  to  look  upon  it  as  a  supernatural  phenomenon.  The 
speed  of  the  barrel  was  evidently  equal  to  the  difference  in  speed  be- 
tween the  upper  and  the  lower  currents. 

In  1773,  Captain  Deslandes  cast  anchor  in  the  Gulf  of  Guinea;  a 
strong  current  running  into  this  bay  prevented  him  from  going  farther 
south.  Deslandes  then  noticed  that  there  was  an  under  counter-cur- 
rent at  the  depth  of  eighty  feet,  and  he  adopted  an  ingenious  plan  for 
availing  himself  of  it.  A  machine,  with  considerable  surface,  was  let 
down  to  the  depth  of  this  submarine  current.  This  was  hurried  along 
with  so  much  force  that  it  towed  the  vessel  at  the  rate  of  one  and  a 
half  miles  per  hour. 

In  the  Antilles  seas  a  vessel  is  sometimes  brought  to  a  halt  even  in 
the  middle  of  a  current. 

In  the  Sound  there  has  long  been  known  to  exist  both  an  upper  and 
an  under  current. 

The  mean  temperature  of  the  surface  of  the  sea  differs  but  little 
from  that  of  the  air,  so  long  as  warm  currents  do  not  add  their  influ- 
ence. In  the  tropics  it  appears  that  the  surface  of  the  water  is  rather 
warmer  than  that  of  surrounding  air. 

An  examination  of  the  temperature  at  the  surface  at  various  depths 
gives  the  following  results : 

1st.  In  the  tropics  the  temperature  diminishes  with  depth. 

2d.  In  the  polar  seas  it  augments  with  increased  depth. 

3d.  In  the  temperate  seas,  included  between  30°  and  70°  latitude, 
the  temperature  decreases  in  a  smaller  degree  as  the  latitude  gets  high- 
er, and  beyond  degree  70  begins  to  increase. 


296  THE  ATMOSPHERE. 

There  exists,  then,  a  zone  the  temperature  of  which  is  almost  sta- 
tionary, from  its  surface  down  to  a  great  depth. 

It  is  scarcely  possible  to  doubt  that  the  currents  caused  by  the  dif- 
ference in  pressure  which  strata  of  the  same  level  are  subject  to,  at  the 
equator  and  toward  the  poles,  contribute  materially  to  this  distribution 
of  heat.  It  seems  certain  that  there  is,  as  a  rule,  a  surface  current 
which  carries  the  warm  waters  of  the  tropics  toward  the  polar  seas, 
and  an  under-current  which  takes  back  from  the  poles  to  the  equator 
the  frigid  water  of  the  polar  regions ;  but  the  direction  and  intensity 
of  these  currents  are  modified  by  a  number  of  causes,  which  depend 
upon  the  depth  of  the  sea  basins,  their  shape,  and  the  influence  of  winds 
and  tides.  In  very  deep  water  there  is  a  uniform  temperature  of  39°, 
which  corresponds,  as  physical  science  has  proved,  to  the  maximum  of 
the  density  of  water.  This  temperature  exists  at  the  equator  at  a  depth 
of  7200  feet.  In  the  polar  regions,  where  the  water  is  colder  upon  the 
surface,  this  same  temperature  is  met  with  at  a  depth  of  4600  feet.  The 
isothermal  lines  of  39°  form  the  demarkation  between  the  zones  where 
the  surface  of  the  sea-water  is  colder,  and  those  where  it  is  hotter  than 
the  stratum  which  marks  39°. 

Lastly,  the  quantity  of  salt  in  the  waters  of  the  ocean  differs  accord- 
ing to  the  points  of  the  globe,  and  is  unquestionably  an  important  ele- 
ment in  the  density,  and,  consequently,  in  the  actual  formation,  of  mari- 
time currents. 


THE  VARIABLE  WINDS.  297 


CHAPTER  III. 

THE  VARIABLE  WINDS:  THE  WIND  IN  OUR  CLIMATES  —  MEAN  DIREC- 
TIONS IN  EUROPE  AND  IN  FRANCE — RELATIVE  FREQUENCY  OF  THE 
DIFFERENT  WINDS — RISE  OF  THE  WINDS  ACCORDING  TO  THE  TIMES 
AND  PLACES— MONTHLY  AND  DIURNAL  VARIATION  IN  INTENSITY. 

HAVING  observed  the  regular  and  periodical  currents  of  the  atmos- 
phere and  the  seas,  let  us  now  consider  the  irregular  winds  which  blow 
over  our  climates.  These  latter  are  only  apparently  irregular,  for  in 
nature  there  is  no  such  thing  as  chance,  and  each  molecule  of  air  that 
changes  position  is  obeying  laws  as  absolute  as  those  which  regulate  the 
worlds  of  space.  We  will  endeavor  to  throw  some  light  upon  the  mul- 
titude of  winds  which  succeed  each  other  in  our  regions,  and  to  ascer- 
tain the  causes  which  set  them  in  motion. 

Beyond  the  changing  limits  within  which  blow  the  trade-winds  and 
the  periodical  breezes  of  the  two  hemispheres,  the  temperate  zones  are 
the  seat  of  variable  winds.  Europe,  for  instance,  is  entirely  subject  to 
that  regime,  and  the  masses  of  air  float  off  sometimes  in  one  direction, 
sometimes  in  another.  Now  and  then  one  kind  of  wind  will  prevail  for 
weeks  together;  sometimes,  on  the  other  hand,  the  wind  will  blow  from 
two  or  three  different  points  of  the  compass  in  as  many  hours ;  some- 
times, again,  the  air  remains  calm,  and  there  is  not  a  breath  of  wind  to 
agitate  even  the  foliage  of  the  poplar-tree.  Thus  the  instrument  used 
to  indicate  the  direction  of  the  winds  in  our  climates,  the  weather-cock, 
has  long  been  taken  to  signify  inconstancy. 

Nevertheless,  even  inconstancy  has  a  cause,  and  is  often  more  appar- 
ent than  real.  The  winds  in  our  climates,  which  seem  to  us  so  capri- 
cious and  variable,  leave  behind  them  a  trace  of  the  laws  which  they 
follow. 

We  have  seen  that  the  upper  trade  -  winds,  which  travel  from  the 
equator  to  the  pole,  modify  their  primitive  direction  from  north  to  south 
in  our  hemisphere,  and  veer  gradually  to  the  south-west  as  they  reach 
higher  latitudes.  They  lose  at  the  same  time  both  in  velocity  and  heat, 
and  gradually  come  nearer  to  the  ground.  About  30°  latitude  they  are 
almost  on  a  level  with  the  surface.  This  south-west  wind,  in  fact,  pre- 


298 


THE  ATMOSPHERE. 


vails  throughout  Europe.  Thus,  amidst  the  variety  of  winds,  we  al- 
ready find  that  there  is  one  which  is  regular,  since  it  is  no  other  than 
the  upper  trade-wind  which  has  descended  thus  far,  and  which  occupies 
the  largest  place  in  the  meteorology  of  our  climates. 

We  have  seen  that  the  great  oceanic  current,  the  Gulf  Stream,  reach- 
es the  coasts  of  Europe  from  the  south-west.  The  air  circulates  in  the 
same  direction,  and  increases  still  further  the  inflection  of  the  upper 
trade-winds,  or,  to  speak  more  correctly,  it  is  the  same  equatorial  aerial 
and  maritime  current  turned  off  in  a  south-west  direction  by  the  rota- 
tion of  the  earth. 

To  ascertain  precisely  the  direction  of  the  winds,  it  is  necessary  to 
keep  an  account  of  the  time  during  which  each  wind  has  prevailed,  tak- 
ing a  supposititious  total  upon  which  the  calculation  is  based.  Thus,  for 
instance,  let  us  suppose  that  the  south-west  wind  has  been  blowing  for  a 
little  more  than  ninety  days  of  the  year;  it  would  be  put  down  that  it 
had  prevailed  for  a  quarter  of  the  whole  time.  If  the  total  1000  be 
taken  to  signify  this  time,  250  would  be  placed  to  the  account  of  the 
south-west  wind  (that  is,  if  it  had  been  blowing  for  ninety-one  days  and 
seven  hours,  which  is  exactly  a  quarter  of  a  year).  In  the  same  way 
all  directions  indicated  by  the  vane  would  be  similarly  put  down,  and 
thus  we  should  obtain  a  comparative  table  giving  the  average  result  for 
a  long  series  of  years. 

This  plan  has  been  adopted  in  Europe  for  many  years,  and  the  fol- 
lowing table  will  show  the  result  of  the  observations  made.  It  indi- 
cates a  decided  preponderance  of  a  south-westerly  wind  over  the  Euro- 
pean continent  and  even  North  America : 

RELATIVE  FREQUENCY  OF  DIFFERENT  WINDS. 


N. 

N.K 

E. 

S.E. 

S. 

S.W. 

W. 

N.W. 

Mean 
Direction  of 
the  Wind. 

Mean 
Force  of 
the  Wind. 

France  
England  
Germany  
Denmark  
Sweden  
Russia  
N.  America.  . 

126 
82 
84 
65 
102 
99 
96 

140 
111 
98 
98 
104 
191 
116 

84 
99 
119 
100 
80 
84 
49 

76 
81 
87- 
129 
110 
130 
108 

117 
111 
97 
62 
128 
98 
123 

192 

225 

155 
171 
198 

110 

120 
131 
156 
106 
192 

S.  88°  W. 
S.  66°  W. 
S.  76°  W. 
S.  62°  W. 
S.  50°  W. 
N.87°W. 
S.  86°  W. 

133 

198 
177 
170 
200 
167 
182 

185 
198 

161 
159 
166 
101 

210 

143 
197 

210 

It  will  be  seen  that  the  south-west  is  the  prevailing  wind.  By  adding 
up  the  numbers  set  down,  as  they  run  horizontally,  the  total  will  be 
found  to  be  1000 ;  thus  in  France  the  south-west  wind  blows  -^,  or 


THE  VARIABLE  WINDS.  299 

nearly  a  fifth  of  the  whole  time.  The  proportion  is  greater  still  in  En- 
gland. By  adding  together  the  west  and  the  south,  it  will  be  seen  that 
the  continuance  of  wind  from  this  quarter  amounts  to  nearly  one-half 
of  the  prevailing  winds ;  -f^V  in  France,  and  -^V  in  England.  The 
careful  observations  taken  at  Brussels  and  in  various  parts  of  Belgium 
since  1830  show  a  like  preponderance  to  exist  there.  The  prevailing 
wind  is,  indeed,  just  S.  45°  W.  In  Russia  there  is  a  greater  variety, 
owing  to  its  distance  from  the  ocean. 

Thus  we  are  under  the  benign  influence  of  the  equatorial  current. 
But,  if  the  return  trade-winds  reach  so  far,  and  even  to  the  pole,  the 
lower  polar  current,  which  conveys  the  cold  air  from  north  to  south,  and 
forms  in  the  tropics  the  north-easterly  trade-winds,  must  also  have  its 
influence  upon  our  regions.  It  must  pass  us  somewhere  on  its  way 
from  the  pole  to  the  equator;  and  if  the  air  which  travels  from  the 
equator  to  the  pole  did  not  return,  there  would  cease  to  be  any  atmos- 
phere at  all  in  the  tropics.  Now,  let  us  study  for  a  moment  the  pre- 
ceding table  of  the  relative  frequency  of  the  winds.  The  maximum  is 
to  the  south-west,  as  is  shown  by  the  figures  underlined,  whence  the  to- 
tals become  smaller  and  gradually  swell  again,  giving  a  second  max- 
imum in  the  shape  of  a  KB.  wind.  That  is  our  polar  curr'ent.  The 
N.E.  wind  forms  the  -^  of  the  winds  in  France,  ^V  of  those  in  En- 
gland, and  -r^nr  in  Russia. 

There  exist,  therefore,  in  our  hemisphere  two  general  directions  of 
winds.  Now  it  is  the  equatorial,  now  the  polar  current  which  predom- 
inates. The  first  is  warm  and  moist,  the  latter  cold  and  dry.  Each  has 
an  opposite  influence  upon  the  productions  of  the  soil,  and  the  state  of 
the  crops  depends  in  a  great  measure  upon  the  epoch  and  continuity  of 
their  prevalence. 

The  S.W.,  W.,  and  S.  winds  on  the  one  hand,  the  N.E.  and  N.  winds 
on  the  other,  constitute  the  general  primitive  winds  to  which  our  regions 
are  subject.  All  the  other  directions  of  the  wind  are  due  to  these  two 
currents,  and  for  the  following  reasons : 

If  the  two  currents  are  blowing  in  proximity  to  each  other,  each  oc- 
cupying a  certain  extent,  as  they  are  proceeding  in  opposite  directions, 
there  must  exist,  about  the  limit  which  separates  them,  whirlwinds  and 
circular  blasts  engendered  by  the  action  of  the  two  currents  of  air. 
These  circular  blasts  will  revolve  from  N.E.  to  S.W.  at  the  tangent  of 
the  polar  current,  from  S.W.  to  N.E.  at  the  tangent  of  the  equatorial 
current 


300  THE  ATMOSPHERE. 

As  an  instant's  reflection  will  show,  this  is  a  simple  horizontal  move- 
ment like  that  of  a  grinding-stone.  Each  point  in  the  circumference  of 
this  grinding-stone  will  have  its  own  direction,  since  we  are  supposing 
that  this  mass  revolves  in  its  entirety.  It  would  be,  in  fact,  a  zone  of 
variable  winds  which  would  be  liable  to  change  its  place  under  the  in- 
fluence of  the  two  great  currents  from  which  it  springs,  and  which  them- 
selves vary  in  position,  extent,  and  intensity.  Here  we  have  one  cause 
for  the  change  of  wind  which  is  almost  constant  (since  the  two  currents 
are  always  in  existence),  and  which  must  be  multiplied  to  a  vast  extent. 
There  is  a  second  and  not  less  important  cause. 

There  is  a  constant  difference  of  temperature  in  the  various  regions 
of  the  same  country.  In  one  place  there  is  water,  in  another  land ;  here 
deserts,  there  forests;  at  one  point  low  and  sultry  plains,  at  another 
bleak  table-lands.  These  differences  of  temperature  modify  our  two 
currents  on  their  passage  through  them.  A  cloudy  sky  is  favorable  to 
the  progress  of  the  one,  and  arrests  the  march  of  the  other.  Thus  par- 
tial winds  spring  out,  like  lateral  branches,  from  the  trunks  of  the  two 
great  trees  which  are  lying  prostrate. 

A  third  cause  must  be  superadded  to  the  above — the  protuberances 
upon  the  land.  The  general  currents  which  pass  over  a  chain  of  mount- 
ains do  not  blow  with  the  same  regularity  that  they  do  in  the  plains. 
In  fact,  the  winds  must  be  all  the  more  unequal  in  their  successive 
blasts  in  proportion  as  the  surface  over  which  they  sweep  is  uneven. 
The  same  aerial  surface  which  moves  over  the  waters  with  the  uniform- 
ity of  a  vast  river,  loses  the  regularity  of  its  movement  when  it  is  inter- 
rupted in  its  course  by  the  protuberances^of  the  soil.  At  the  foot  of  the 
Swiss  mountains,  and  especially  around  Geneva,  where  the  ground  is 
very  uneven,  the  alterations  in  the  force  of  the  wind  are  so  great  that 
the  anemometer  sometimes  shows  a  variation  in  intensity  from  one  to 
three.  In  the  lofty  ravines  of  the  Alps  it  often  happens,  even  in  the 
midst  of  the  fiercest  storms,  that  the  atmosphere  is  at  intervals  perfectly 
calm.  Even  in  the  countries  which  are  not  very  hilly,  and  in  the  plains 
studded  with  houses  and  plantations,  the  wind  does  not  blow  with  the 
regularity  of  the  trade-winds  at  sea,  but  advances  with  a  succession  of 
blasts,  each  of  which  represents  a  victory  of  the  atmospheric  current 
over  some  obstacle  upon  the  plain. 

At  the  level  of  the  ground  the  wind  is  always  intermittent,  whereas 
in  the  heights  of  the  air  it  almost  always  proceeds  with  the  regular  and 
majestic  motion  of  a  river. 


THE  VARIABLE  WINDS.  30} 

Thus  laws  regulate  these  minor  changes  as  well  as  the  general  move- 
ment of  circulation.  We  may  now  consider  whether  there  is  any  law 
as  to  the  succession  of  winds. 

Let  us  revert  to  the  first  cause  of  change  dealt  with  above.  As  a 
rule,  our  hemisphere  is  divided  into  large  oblique  bands  composed  of 
masses  of  air  running  in  an  inverse  direction,  some  toward  the  poles, 
others  toward  the  equator.  These  bands  shift  their  position  around  the 
globe,  so  that  at  one  moment  the  polar  wind,  at  another  moment  the 
tropical  wind,  will  prevail  in  the  same  place;  but  there  is  always  a 
compensating  balance  between  these  atmospheric  currents,  and  the  wind, 
which  is  neutralized  or  repelled  in  one  part  of  the  hemisphere,  is  soon 
felt  at  some  other  point.  As  long  as  the  struggle  between  the  two 
masses  of  air  animated  by  opposing  movements  continues,  the  vicissi- 
tudes of  the  conflict  and  the  general  preponderance  of  one  of  the  winds 
cause  a  temporary  modification  in  the  march  of  the  air,  and  make  the 
vane  turn  toward  the  different  points  of  the  horizon.  It  is  from  the  en- 
counter of  the  two  regular  winds  that  chiefly  arises  the  apparent  irregu- 
larity of  the  whole  atmospheric  system. 

Although  the  struggle  between  the  two  aerial  streams  is  continually 
going  on  at  one  point  or  another,  nevertheless  they  are  not  of  equal 
force,  and  one  of  them  always  obtains  the  mastery  after  a  more  or  less 
prolonged  period  of  resistance.  This  wind,  which  proves  the  superior 
in  impulse,  is  the  back  current  that  has  come  down  from  a  great  eleva- 
tion, and  reaches  the  level  of  the  ground  outside  the  zone  of  the  trade- 
winds.  The  atmospheric  currents  coming  from  the  equator  naturally 
incline  toward  the  east,  whence  it  results  that,  in  the  northern  hemi- 
sphere, the  majority  of  the  winds  are  from  the  west. 

Many  centuries  ago  the  savants  ascertained  that  in  the  northern  hemi- 
sphere the  normal  succession  of  the  winds  is  from  south-west  to  north- 
east by  west  and  north,  and  from  north-east  to  south-west  by  east  and 
south.  This  is  a  rotatory  movement  analogous  to  that  which  the  sun 
seems  to  describe  in  the  sky  when,  after  rising  in  the  east,  it  travels 
westward,  developing  its  vast  curve  around  the  zenith.  Aristotle,  in 
his  " Meteorology,"  wrote,  more  than  two  thousand  years  ago,  "When 
a  wind  ceases  to  give  place  to  another,  the  direction  of  which  is  next  in 
order  to  that  of  the  former,  the  change  always  takes  place  with  the  sun." 
Since  the  time  of  the  great  Greek  naturalist,  several  authors  enumerated 
by  Dove  have  re-affirmed  this  fact  of  the  regular  rotation  of  the  winds, 
which  was,  indeed,  known  to  sailors  in  the  earliest  ages.  Dove  was  the 


302  THE  ATMOSPHERE. 

first  who  collected  the  scattered  proofs  of  this  generally  accredited  the- 
ory, and  transformed  the  primitive  hypothesis  into  a  scientific  certainty. 
It  no  longer  admits  of  any  doubt  that,  in  the  northern  hemisphere,  the 
winds  generally  succeed  each  other  in  the  following  order :  S.W.,  W., 
N.W.,  K,-N.K,  E.,  S.E.,  S.,  S.W. 

In  the  southern  hemisphere  the  normal  rotation  of  the  aerial  currents 
is  exactly  the  opposite.  Thus,  as  E.  Eeclus  remarks,  the  procession  of 
the  winds  in  each  of  the  two  hemispheres  coincides  with  the  apparent 
march  of  the  sun,  which,  so  far  as  Europe  is  concerned,  describes  its 
daily  course  to  the  south  of  the  zenith,  and,  in  Australia,  passes  to  the 
north  of  it.  Such  is  the  regular  order  which  Dove  termed  the  law  of 
gyration,  but  which  is  generally  called  after  the  name  of  its  discoverer. 

I  have  noticed  in  my  aerial  travels  a  gyratory  deviation,  which  shows 
that  the  wind  can  not  extend  in  a  straight  line  when  it  spreads  over  a 
great  area,  but  inclines  in  the  direction  indicated  by  the  above  theory. 

Immersed  in  the  atmospheric  current  which  bears  him  along,  the 
aeronaut  is  placed  in  the  most  favorable  position  imaginable,  both  for 
ascertaining  the  continuous  direction  of  the  current  and  for  measuring 
its  speed.  Upon  each  occasion  I  took  care  to  trace  accurately  on  a  map 
of  France  or  Europe  the  aerial  line  taken  by  the  balloon,  which  is  done 
with  extreme  ease  when  the  sky  is  clear,  and  which  may  always  be  ob- 
tained even  with  a  cloudy  sky,  either  by  availing  one's  self  of  the  mo- 
mentary breaks  or  by  descending  every  now  aud  then  below  the  clouds. 

The  balloon  marks  so  accurately  the  direction  and  speed  of  the  cur- 
rent, that  the  first  sensation  in  navigating  the  air  is  that  of  being  com- 
pletely at  a  stand-still.  It  is  a  peculiar  and  always  surprising  impres- 
sion experienced  when,  traveling  along  with  the  velocity  of  the  wind, 
one  feels  neither  the  slightest  breath  of  air  nor  the  least  movement,  even 
when  hurriedly  carried  off  into  space  by  the  most  violent  tempest.  I 
never  felt  but  once  any  thing  like  a  breeze.  This  was  on  the  15th  of 
April,  1868,  and  then  only  for  a  few  minutes.  This  I  attribute  to  the 
fact  that  the  balloon,  which  was  traveling  at  the  rate  of  thirty-four  miles 
an  hour,  had  reached  a  region  where  the  air  was  shifting  its  position  less 
rapidly.  One  capital  fact  is  brought  to  light  by  the  aerial  lines  which 
I  have  traced,  and  that  is,  that  these  routes  all  incline  in  the  same  di- 
rection, by  virtue  of  a  general  gyratory  deviation. 

The  actual  direction  of  a  wind  is  the  most  easily  observed  of  its  char- 
acteristics. To  ascertain  it,  we  suppose  the  horizon  to  be  divided  into 
four  equal  sections  by  two  diameters  perpendicular  to  one  another,  one 


THE   VARIABLE  WINDS.  gQg 

running  from  south  to  north,  the  other  from  east  to  west.  The  points 
at  which  the  diameters  intersect  the  horizon  are  called  the  four  cardinal 
points.  But  they  would  not  of  themselves  suffice,  for  it  is  necessary  to 
have  a  number  of  intermediate  directions.  These  are  indicated  by  oth- 
er diameters,  which  divide  the  horizon  into  sixteen  equal  parts;  and 
thus  we  obtain  the  indications  of  the  wind  at  as  many  different  direc- 
tions, called,  starting  from  K  round  by  E.,  N.N.E. ;  N.E. ;  E.KE. ;  E. ; 
E.S.E. ;  S.E.;  S.S.E. ;  S. ;  S.S.W. ;  S.W.;  W.S.W. ;  W. ;  W.N.W.; 
N.W.;  N.N.W.;  K 

When  the  points  of  the  compass  are  known,  and  objects  are  affected 
by  the  movement  of  the  air,  it  is  easy  to  ascertain  the  direction  of  the 
wind ;  but  often  recourse  is  had  to  an  instrument  which  is  no  doubt 
the  oldest  of  those  used  in  meteorology,  viz.,  the  weather-vane.  This 
simple  apparatus  consists  of  a  metal  plate,  generally  of  tin  or  zinc,  cut 
into  a  figure  of  some  kind,  and  turning  upon  a  rod,  to  which  is  attach- 
ed a  horizontal  cross  with  the  letters  N.,  S.,  W.,  E.,  at  its  extremities. 
The  weather-vane  is  placed  upon  the  highest  part  of  a  building,  and  in 
by-gone  days  no  house  of  moderate  size  was  deemed  complete  without 
it.  Exposed  to  the  weather,  it  becomes  corroded,  and  ceases  to  follow 
implicitly  the  impulsion  of  the  winds.  Sometimes  the  rod  gets  out 
of  order,  and  the  vane  inclines  to  one  side.  Its  indications  are  not 
worth  consideration  unless  they  are  verified  from  time  to  time,  and  the 
vane  is  situated  beyond  the  influence  of  obstacles  which  obstruct  the 
free  passage  of  the  wind.  It  is  not  a  rare  occurrence  for  the  atmos- 
phere to  be  influenced  by  several  different  currents,  one  superposed 
upon  the  other.  In  this  case,  the  principal  current  —  that  which,  so 
to  speak,  governs  the  weather — is  generally  placed  at  a  considerable 
height,  even  when  it  is  not  the  highest  of  all ;  it  is  discovered  by  the 
motion  of  the  clouds.  This  is  the  best  and  surest  indication  of  the 
direction  of  the  wind. 

As  the  mass  or  density  of  the  air  only  varies  within  very  restricted 
limits,  the  force  of  the  wind  depends  almost  entirely  upon  its  speed, 
and  varies  as  the  square  of  its  velocity,  or  very  nearly.  The  terms 
"  force  of  the  wind  "  and  "  velocity  of  the  wind  "  are  therefore  almost 
identical.  To  measure  the  speed,  an  apparatus,  called  an  anemometer, 
is  used.  One  of  those  most  frequently  in  use  is  that  by  Dr.  Robinson, 
of  Armagh.  This  instrument  is  composed  of  a  vertical  axis  support- 
ing four  horizontal  radii  of  the  same  length,  crossing  at  right  angles, 
and  at  the  extremities  of  which  are  four  hollow  half-spheres. 


304 


THE  ATMOSPHERE. 


A  moment's  reflection  will  suffice  to  make  it  clear  that  the  wind  is 
always  pressing  against  two  concave  and  two  convex  half-spheres.  As 
it  has  more  power  over  the  former  than  over  the  latter,  it  causes  a  ro- 
tatory motion,  and  the  number  of  revolutions  which  the  half-spheres 
make  is  proportional  to  the  velocity  of  the  wind.  The  number  three 
represents  with  approximate  accuracy  the  relation  which  exists  be- 
tween the  horizontal  movement  of  the  air  and  the  horizontal  move- 
ment of  the  half-spheres.  Thus,  by  measuring  the  circumference  of 
the  circle  which  the  centre  of  one  of  the  demi-spheres  describes,  and 
by  multiplying  half  its  length  by  three,  we  obtain  the  distance  traveled 
by  the  wind  for  each  revolution  of  the  apparatus. 

The  monthly  averages  of  each  wind  referred  to  eight  points  of  the 
compass,  as  found  from  sixty  years'  observations  at  the  Observatory  at 
Paris  (1806-1866),  are  as  follows: 

PKOPORTION  UPON  10,000  WINDS. 

TheN '. '. 1039 

N.W....  ..  1084 


W.... 
S.W. 

s 

S.E.. 
E 

N.E. 


1782 


694 
1191 


These  numbers  show  the  dominant  winds  to  be  S.W.  and  S. 

The  monthly  averages  of  the  winds  at  London  show  a  prevalence  of 
south-westerly  winds  to  an  even  more  marked  extent  than  in  Paris. 
The  result  of  observations  taken  for  twenty  consecutive  years  at  the 
Greenwich  Observatory,  which  I  have  received  from  Mr.  Glaisher,  the 
director  of  the  meteorological  service  there,  gives  the  following  averages 
of  the  relative  frequency  of  each  wind  (see  Fig.  58) : 

The  N.       wind  blows  on  an  average  for  41  days. 


N.E. 

E. 

S.E. 

S. 

S.W. 

W.         " 

N.W.       " 

Days  of  complete  calm 


48 
22 
20 
34 
104 
38 
24 
34 

365 


THE  VARIABLE  WINDS. 


305 


The  average  direction  of  the  winds  at  Brussels  gives  the  same  result 
(see  Fig.  59),  and  we  have  already  remarked  the  predominance  of  the 
equatorial  current  in  the  study 
of  the  general  mass  of  observa- 
tions taken  throughout  Europe. 

It  seems  certain  that  the  wind 
is  propagated  not  only  by  im- 
pulsion, but  by  aspiration.  This 
second  mode  deserves  attention 
because  it  furnishes  important 
data  as  to  the  cause  of  the 
movement.  Franklin  appears 
to  have  been  the  first  to  ob- 
serve this  fact.  He  mentions  in 
one  of  his  letters  that,  when  at- 
tempting to  watch  an  eclipse  of 
the  moon  at  Philadelphia,  he  s< 

was    prevented    from    doing    SO  FIS-  58.— Average  annual  prevalence  of  the  different 
,  .  .  winds  at  London. 

by  a  hurricane  from  the  north- 
east, which  took  place  at  about  seven  in  the  evening,  and  was  fol- 
lowed, as  is  usually  the  case,  by  clouds  which  obscured  the  whole  sky. 
He  learned  to  his  surprise,  some  time  afterward,  that  at  Boston,  which 

is  about  400  miles  to  the  north-east 
of  Philadelphia,  the  storm  had  not 
commenced  until  11  P.M.,  long  after 
the  first  phases  of  the  eclipse  had 
been  observed ;  and,  by  a  compari- 
son of  the  various  accounts  collect- 
ed in  different  colonies,  Franklin 
remarked  that,  according  as  the 

Fig.  59,-Average  annual  prevalence  of  the  differ- Place    was    farther    north,    the    later 

ent  winds  at  Brussels.  was  the  hour  at  which  this  north- 

easterly tempest  occurred  there,  and  that  thus  the  wind  was  blowing  in 
one  direction  and  was  advancing  progressively  in  another. 

Since  that  time  a  great  number  of  tempests  have  been  remarked, 
which  presented  this  peculiarity  in  respect  to  their  direction.  Never- 
theless, in  nearly  every  case,  the  wind  advances  in  the  direction  toward 
which  it  is  blowing. 

The  terrible  storm  from  the  south-west,  which  occurred  on  Novem- 

20 


306  THE  ATMOSPHERE. 

ber  29, 1836,  passed  over  London  at  10  A.M.,  the  Hague  at  1  P.M.,  Am- 
sterdam at  1-30  P.M.,  Emden  at  4  P.M.,  Hamburg  at  6  P.M.,  and  Stettin 
at  9-30  P.M.  It  traveled,  therefore,  in  the  same  direction  as  that  in 
which  it  was  blowing,  and  took  ten  hours  to  reach  Stettin  from  London. 

The  following  is  a  general  sketch  of  the  prevailing  distribution  of 
wind  over  the  surface  of  the  globe : 

Suppose  a  ship  to  start  from  the  Arctic  Polar  Circle  for  the  equator, 
to  cross  it,  and  proceed  onward  to  the  Southern  Arctic  Circle,  it  will 
meet  with  the  following  succession  of  winds : 

1st.  At  the  outset,  it  navigates  in  the  region  of  south-westerly  winds 
or  of  the  northern  anti-trade-winds,  so  called  because  they  blow  in  an 
opposite  direction  to  the  trade- winds  of  their  hemisphere. 

2d.  After  having  crossed  the  parallel  of  latitude  50°,  and  until  it 
reaches  that  of  35°,  it  encounters  the  zone  of  partially  western  winds, 
in  which  south-west  predominates,  and  in  which  the  north-easterly  cur- 
rent also  prevails  over  the  other  winds. 

3d.  Between  K  latitudes  40°  and  46°  there  is  a  region  where  the 
winds  are  very  variable,  and  where  there  are  calms.  The  winds  blow, 
in  the  course  of  the  year,  in  equal  proportions  from  the  four  quarters 
during  three  months. 

4th.  To  the  west  winds,  which  have  predominated  thus  far,  succeeds 
the  calm  region  of  the  Tropic  of  Cancer,  then  that  of  the  trade-winds 
which  conduct  the  vessel  to  the  latitude  of  10°  north,  where  it  reaches 
the  zone  of  equatorial  calm,  which  is  only  5°  in  breadth. 

5th.  From  5°  north  to  30°  south  the  south-easterly  trade-winds  pre- 
vail. 

6th.  Then  succeeds  the  calm  zone  of  the  Tropic  of  Capricorn,  analo- 
gous to  that  of  the  Tropic  of  Cancer. 

7th.  From  S.  latitude  35°  to  40°  there  prevail,  as  a  rule,  westerly 
winds,  which  sometimes  veer  to  N.W.  and  to  S.W. 

8th.  Lastly,  the  vessel  reaches  at  S.  latitude  40°  the  southerly  anti- 
trade-winds, which  have  a  north-westerly  direction,  and  prevail,  as  far 
as  observations  in  the  direction  of  the  Southern  Pole  have  extended. 

If  we  now  consider  the  intensity  of  the  wind,  we  notice  that  its  varia- 
tion, apparently  so  irregular,  is  dependent,  like  every  thing  else,  upon 
the  movements  of  the  earth,  in  the  seasons  and  in  the  days.  Twenty 
years'  comparisons  made  at  Brussels  show  that  the  wind  is  less  intense 
during  the  longest  days  than  during  the  shortest,  as  in  June  the  indi- 


THE  VARIABLE  WINDS. 


307 


\ 


cations  of  intensity  are  0-832,  and  in  December  1-227.  The  month 
of  September,  however,  seems  to  be  an  exception,  for  it  gives  the 
minimum,  averaging  only  0*804;  but  this  month  is,  in  many  re- 
spects, an  exceptional  one  in  our 
climates. 

It  is,  moreover,  remarkable  that 
during  the  six  months  when  the 
sun  is  below  the  equator  the  force 
of  the  wind  is  above  the  average 
of  the  year ;  whereas,  on  the  con- 
trary, its  force  is  generally  below 
the  average  during  each  of  the 
other  six  months. 

The  intensity  of  the  wind  varies,  too,  according  to  the  time  of  day. 
The  anemometer  at  the  Brussels  Observatory,  which  registers  the  wind 
every  five  minutes,  shows  that  this  diurnal  variation  in  the  inten- 
sity of  the  winds  extends  from 
an  average  of  015  (midnight  to 
4  A.M.)  to  0-21  (10  A.M.),  0-26 
(noon),  0-29  (2  P.M.),  0-28  (4 


Fig.  60.— Monthly  intensity  of  the  winds. 


P.M.),  and  0-23  (6  P.M.)  This  is 
shown  by  Fig.  61. 

Thus  the  wind  is  almost  twice  as  strong  at  2  P.M.  as  in  the  middle 
of  the  night. 

The  time  will  arrive  when  the  march  of  the  variable  winds  in  our 
climates  will  be  ascertained,  just  as  the  general  circulation  of  the  trade- 
winds  and  the  monsoons  in  the  tropical  regions  has  long  been  made 
known.  The  day  will  come,  too,  when  observations  of  the  upper  winds 
will  have  revealed  to  the  meteorologist  the  route  which  they  follow, 
just  as  observations  of  the  planets  have  discovered  to  the  astronomer 
the  orbits  they  describe.  Then  we  shall  be  able  to  tell  the  daily  and 
yearly  direction  of  the  atmospheric  wave  which  passes  over  our  heads. 

The  currents,  the  laws  of  which  we  have  been  studying,  play  a  great 
part  in  nature.  They  favor  the  growth  of  flowers  by  causing  the 
branches  of  the  plants  to  oscillate,  and  blowing  the  seeds  a  long  dis- 
tance. They  renovate  the  air  in  cities,  and  render  northern  climates 
milder  by  supplying  them  with  heat  from  the  south.  Without  wind 
rain  would  be  unknown  in  the  interior  of  continents,  which  would  be 
transformed  into  arid  deserts.  Without  wind  the  earth  would  be  al- 


308  THE  ATMOSPHERE. 

most  uninhabitable,  and  whole  districts  would  become  centres  of  con- 
tagion— vast  cemeteries,  in  fact.  We  have  seen  the  deleterious  effects 
of  air  when  confined.  Man  acts  as  a  deadly  poison  to  man,  as  typhus 
fever  and  plagues  clearly  demonstrate.  The  winds  alone  can  avert 
these  calamities,  by  blowing  away  the  emanations,  by  disseminating 
them  in  the  regions  of  space,  and  substituting  for  vitiated  air  a  fresh, 
salubrious  atmosphere.  Moreover,  it  is  the  same  with  air  as  with  wa- 
ter ;  motion  alone  keeps  it  pure,  whether  because  it  has  a  principle  of 
life  unknown  to  us,  or  because  animalcules,  or  vegetable  and  animal 
debris,  becoming  decomposed  when  at  rest,  spread  their  deleterious 
principles  throughout  a  motionless  atmosphere. 

The  winds  not  only  bring  life  upon  their  blast,  they  may  also  trans- 
mit death  to  countries  where  the  yellow  fever,  the  plague,  or  cholera 
prevails. 

A  distance  of  twenty  leagues  does  not  protect  Rome  from  the  deadly 
air  which  has  blown  over  the  Pontine  marshes.  In  Paris  the  west 
wind  blows  for  seventy  days  in  a  year ;  place  an  Agro  Romano  in  the 
Mayenne,  the  Sarthe,  or  Touraine,  and  the  population  of  Paris  would 
be  decimated  by  intermittent  fever.* 

It  has  been  mentioned  that  in  all  latitudes  similar  to  those  of  Europe, 
and  even  rather  more  southerly,  the  prevailing  wind  is  west,  which 
conveys  to  Europe  the  warm  air  of  the  Atlantic,  and  endows  it  with 
that  unique  climate  which  admits  of  the  cultivation  of  barley  and  other 
cereals  as  far  as  the  North  Cape ;  whereas  in  Greenland,  which  is  de- 
prived of  these  balmy  breezes,  it  never  thaws,  although  this  latter  coun- 
try is  in  about  the  same  latitude  as  the  north  of  Scotland.  The  city 
of  Boston,  in  the  United  States,  is  in  the  same  latitude  as  the  olive- 
growing  districts  of  Spain.  Nevertheless,  during  the  winter  there,  the 
small  lakes  in  the  neighborhood  are  sometimes  frozen  a  yard  deep. 
The  five  great  American  lakes  (which  are,  in  truth,  inland  seas)  freeze 
over,  and  are  traversed  by  temporary  railroads.  What  a  striking  con- 
trast between  the  climate  which  produces  this  ice  and  that  where  the 
olive-oil  and  wine  afford  an  easy  subsistence  to  the  indolent  cultivators 
about  Bordeaux  and  in  Spain !  Yet  the  intelligent  activity  of  the  in- 
habitant of  the  United  States  has  transformed  even  this  ice  into  a  prof- 

*  There  are  at  times  strange  variations  in  the  Bills  of  Mortality  which  can  be  due  to  no 
other  cause  than  the  wind.  Thus,  for  instance,  on  July  26th,  1871,  half  of  the  inhabitants  of 
Paris  were  attacked  by  a  mild  form  of  cholerine.  There  had  been  no  other  perturbation  than 
a  heavy  gale  of  wind  which  raged  all  the  previous  night. 


THE  VARIABLE  WINDS. 


309 


itable  crop,  which  is  exported  to  India  and  the  tropical  regions,  fetch- 
ing a  higher  price  than  that  obtained  for  the  olives  of  the  Asturias. 

Toward  the  centre  of  France  there  exists  the  most  exquisite  climate 
of  the  whole  world,  so  that  if  a  locality  be  selected  somewhere  about 
the  east  of  the  meridian  of  Paris,  it  will  possess  a  more  favorable  cli- 
mate than  any  other  place  in  the  same  latitude. 

Let  us  now  consider  the  influence  which  the  wind  has  on  climatol- 
ogy. The  winds  have  a  dominating  influence  upon  the  distribution 
of  temperature,  as  they  effect  in  different  countries,  according  to  their 
positions  in  respect  to  the  four  cardinal  points,  permanent  modifica- 
tions in  the  climate  which  these  countries  would  otherwise  have.  The 
regime  of  the  winds  leads  to  a  regime  of  temperature  which  is  indis- 
solubly  connected  with  it.  The  currents  of  the  atmosphere  bring  with 
them  the  temperature  of  countries  whence  they  come.  Every  one  may 
have  noticed  that  the  north  wind  is  generally  cold,  and  the  south  wind 
generally  warm.  But  it  would  be  commonplace  to  be  satisfied  with 
these  vague  indications,  and  the  role  of  science  is  to  analyze  facts.  Con- 
sequently, for  many  years  past,  the  temperatures  which  the  thermome- 
ter denotes  for  the  directions  of  the  wind  have  been  carefully  compared, 
and  one  of  the  first  results  was  to  show  that  in  France  the  winds  blow- 
ing from  the  south-east  and  the  south  cause  an  increase  of  5°  or  7°  in 
the  temperature  over  those  which  blow  from  the  opposite  direction.  A 
comparison  of  the  mean  corresponding  temperatures  of  the  different 
winds  throughout  the  various  cities  of  Europe  has  made  it  evident  that 
the  influence  of  the  wind  varies  according  to  places,  as  may  be  seen  by 
the  appended  table : 

INFLUENCE  OF  THE  WINDS  UPON  TEMPERATURE. 


Stations. 

N. 

N.E. 

E. 

S.E. 

S. 

S.W. 

W. 

N.W. 

Differ- 
ences. 

Paris 

Deg. 
52-2 

Deg. 
52'7 

Deg. 

55  '8 

Deg. 
59-2 

Deg. 
59-4 

Deg. 

58-5 

Deg. 
56-1 

Deg. 
53-4 

Deg. 

7'2 

Carlsruhe  
London  
Dublin 

50-9 
45-9 
45  '3 

47-5 
46-6 
46  '6 

50-9 
49-3 

48  '2 

55-6 
51-1 
49-3 

54-5 
52-5 
50-9 

51-6 
51-4 
50-7 

54-3 
50-4 
48'0 

52-2 
47-7 
45-5 

8-1 
6-6 
6-6 

46'4 

45-7 

47'1 

49  '1 

50-0 

50-2 

48-6 

47-1 

4-5 

Zecken  (Silesia)  
Ary  s  (Prussia)  

42-3 
39-4 

43'5 
39-9 

45-7 
38-1 

46-8 
46-2 

49-3 
43-7 

49-1 
43-5 

46-8 
44-6 

44-4 
46-6 

7-0 
8-5 

Reikiawick  (Iceland).  . 
Moscow  

35-1 
34-2 

35-8 
34-5 

41-2 
38-3 

45-0 
39-2 

46-6 
42-8 

38-5 
42-3 

45-9 
41-7 

45-7 
37-9 

11-5 
8'6 

Thus  the  mean  difference  between  the  influence  of  the  warm  and  of 
the  cold  winds  reaches  7°'2  in  Paris,  and  as  much  as  110<5  in  Iceland. 
There  are  often  differences  even  more  marked. 


310 


THE  ATMOSPHERE. 


The  coldest  wind  is  nearly  always  that  which  blows  from  a  direction 
between  north  and  east.  The  warmest  wind  is  nearly  always  from 
S.S.W.  The  farther  one  passes  inland  the  nearer  it  approaches  to 
the  west. 

The  preceding  fact  is  a  confirmation  of  the  meteorological  truth  that 
no  phenomenon  stands  alone :  all  act  and  react  upon  each  other.  No 
sooner  does  the  S.W.  wind  begin  to  blow  than  it  takes  effect  upon  the 
temperature,  not  only  by  its  warmth,  but  by  the  vapor  which  it  brings, 
and  the  condition  of  the  sky  which  is  the  consequence.  In  winter,  the 
moist  west  winds  are  remarkably  warm,  because  they  cover  the  sky  with 
clouds,  and  thus  prevent  loss  of  heat  by  terrestrial  radiation. 

The  winds  affect  not  only  temperature,  but  also  atmospheric  pressure. 

When  the  north  and  north-easterly  winds  are  blowing,  the  barometer 
rises ;  it  falls  when  the  wind  is  from  the  S.  or  the  S.W. 

The  following  is  the  result  of  a  great  many  years'  observations  in  the 
principal  cities  of  Europe,  and  it  shows  very  clearly  the  influence  of  the 
wind  upon  the  reading  of  the  barometer: 

INFLUENCE  OF  THE  DIFFERENT  WINDS  UPON  THE  BAROMETER. 


Winds. 

Paris. 

London. 

Copen- 
hagen. 

Berlin. 

Halle. 

Vienna. 

Stock- 
holm. 

St.  Pe- 
tersburg. 

Moscow. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches. 

N  ;  . 

29-892 

29-890 

30-108 

29-902 

29-753 

29-536 

29-843 

29-919 

29-283 

N.E..    . 

29-938 

29-953 

30-136 

29-921 

29-764 

29-511 

29-910 

30-028 

29-358 

E  

29-830 

29-891 

30-104 

29-902 

29-709 

29-384 

29-819 

30-001 

29-288 

S.E...  . 

29-699 

29-809 

29-898 

29-749 

29-629 

29-461 

29-726 

30-030 

29-219 

S  

29-672 

29-728 

29-920 

29-592 

29-571 

29-451 

29-682 

29-918 

29-171 

S.W..  . 

29-675 

29-755 

29-890 

29-658 

29-619 

29-403 

29-699 

29-949 

29-164 

W  

29-776 

29-847 

29-992 

29-764 

29-619 

29-381 

29-782 

29-911 

29-201 

N.W.... 

29-866 

29-856 

30-096 

29-836 

29-711 

29-520 

29-811 

29-898 

29-228 

Mean  .  .  . 

20-798 

29-858 

30-035 

29-790 

29-694 

29-478 

29-803 

29-963 

29-257 

The  general  result  of  these  researches  is  that  the  barometer  rises 
highest  with  the  wind  between  north  and  east — that  is  to  say,  when  the 
current  is  coldest ;  and  that  its  minimum  elevation  is  when  the  wind  is 
anywhere  between  south  and  west,  the  points  from  which  its  current 
blows  the  warmest.  Analogous  conclusions  have  been  obtained  in  oth- 
er countries.  Thus,  upon  the  eastern  coasts  of  the  United  States  and 
China,  the  barometer  is  generally  highest  when  the  wind  is  in  the  north- 
west— the  coldest  which  prevails  in  those  regions — and,  as  a  rule,  low- 
est when  it  is  in  the  south-east,  the  temperature  being  at  its  maximum 
when  the  wind  is  in  this  direction. 

The  fact  of  the  reading  of  the  barometer  increasing  with  cold  winds, 


THE  VABIABLE  WINDS.  31 1 

and  decreasing  when  the  winds  are  warm,  is  one  that  has  been  made 
evident  wherever  observations  upon  the  point  have  been  taken. 

It  may  be  generally  stated,  so  far  as  our  hemisphere  is  concerned,  that 
the  barometer  reaches  its  maximum  when  the  winds  blow  from  the  north  and 
the  interior  of  continents,  and  its  minimum  when  they  come  from  the  equator 
or  the  sea. 

In  Europe  the  most  rain-bringing  winds  are  those  between  south  and 
west,  and  the  driest  those  between  north  and  east :  this  is  the  reason 
why  it  rains  oftener  when  the  barometer  is  low  than  when  it  stands 
high. 

Just  as  the  winds,  according  to  the  direction  whence  they  come,  in- 
fluence the  temperature  and  the  pressure  of  the  air,  the  reading  of  the 
thermometer  and  the  barometer,  so  do  they  affect  humidity,  announcing, 
bringing  on,  and  keeping  off  rain.  Daily  experience  tells  us  that  the 
air  has  not  always  the  same  degree  of  moisture  irrespective  of  the  direc- 
tion of  the  wind.  When  the  farmer  desires  to  harvest  his  hay  or  corn, 
when  the  laundress  puts  out  her  linen  to  dry,  their  task  is  accomplished 
far  more  rapidly  with  an  easterly  than  with  a  westerly  wind.  Certain 
dyeing  operations  can  only  be  attempted  with  the  wind  in  the  east.  In- 
structive as  these  observations  may  be,  they  can  not,  however,  provide 
us  with  rigorous  and  unchanging  laws. 

The  air  always  contains,  in  addition  to  the  gases  of  which  it  is  com- 
posed, a  certain  quantity  of  vapor  of  water,  and  this  element  plays  a 
principal  part  in  the  absorption  and  distribution  of  heat  over  the  sur- 
face of  the  globe. 

It  would  be  to  the  highest  degree  important  to  be  able  to  ascertain 
numerically  the  quantity  of  vapor  which  exists  in  the  several  regions 
of  the  globe.  The  life  of  plants  and  of  animals,  the  nature  of  the  land- 
scape, are  dependent  upon  this  element  as  well  as  upon  temperature : 
the  dryness  and  the  humidity  of  the  air  have  the  greatest  influence  upon 
the  development  of  disease.  What  we  do  know  is  that  the  air  above 
all  the  seas  is  saturated  with  vapor  of  water. 

The  farther  inland,  the  drier  the  air  becomes:  nevertheless,  after 
long-continued  rain,  it  is  at  times  saturated  with  moisture  overland,  be- 
cause soft  water  vaporizes  more  readily  than  salt  water.  But,  generally 
speaking,  the  quantity  of  vapor  of  water  contained  in  the  air  varies  ac- 
cording to  the  country ;  and  there  are  regions — the  deserts  of  Africa 
and  Asia  and  the  steppes  of  Siberia,  for  instance — where  there  is  not 
the  slightest  evaporation  from  the  soil,  and  where  the  air  is  dry  in  the 


312 


THE  ATMOSPHERE. 


extreme.     The  winds  which  come  from  the  sea  bring  humid  air ;  those 
which  blow  from  the  land  bring  dry  air. 

The  quantity  of  vapor  with  which  the  air  may  be  laden  varies,  ac- 
cording to  temperature,  in  the  following  proportions : 


At  14° 

30 
41 
49° 
56° 


100° 


a  cubic  foot  of  air  is  saturated  with  water  by  the  weight  of  one  grain. 

two  grains, 
three  grains, 
four  grains, 
five  grains, 
seven  grains, 
eleven  grains, 
fourteen  grains. 
'  "  twenty  grains. 


At  212°,  the  air  is  capable  of  absorbing  a  quantity  of  vapor  of  water 
equal  to  its  own  volume ;  the  tension  of  the  water  becomes  equal  to 
that  of  the  air ;  it  boils ;  and  the  pressure  of  the  vapor  is  equal  to  one 
atmosphere. 

Thus  the  hotter  the  air  the  more  it  can  contain  of  water  in  a  state  of 
invisible  vapor.  Let  us  suppose  a  cubic  foot  of  air  to  be  saturated  with 
vapor  at  100° :  it  contains  twenty  grains.  Now,  if  a  current  of  cold  air 
sets  in  and  reduces  it  to  30°,  as  it  can  only  now  contain  two  grains,  it  is 
obliged  to  part  with  about  eighteen  grains  of  water.  This  condensation 
would  lead  to  diurnal  rains  if  cold  currents  were  to  encounter  daily  sat- 
urated masses  of  air. 

The  quantity  of  vapor  is  at  its  minimum  when  the  wind  is  blowing 
between  N.  and  N.B.;  it  increases  when  the  wind  is  in  the  E.,  the  S.E., 
and  the  S.,  and  attains  its  maximum  when  the  vane  points  to  S.  and 
S.W.,  diminishing  again  when  the  breeze  is  from  the  W.  and  the  N.W. 
The  cause  of  these  differences  is  very  simple.  Before  reaching  us,  the 
west  winds  pass  over  the  Atlantic,  and  are  loaded  with  vapor,  whereas 
those  which  blow  from  the  east  come  from  the  interior  of  Europe  and 
Asia.  These  vapors  resolve  themselves  into  rain  when  the  west  winds 
reach  France ;  but  this  water  is  vaporized  almost  immediately,  and  the 
result  is  that  these  winds  continue  to  be  more  charged  with  vapor  than 
those  which  come  from  the  east  The  W.S.W.  wind,  blowing  both 
from  the  sea  and  from  warmer  countries,  is  capable  of  containing  a 
larger  quantity  of  vapor  of  water  than  the  west  wind,  which  is  colder. 
This  is  not  the  case  in  regard  to  relative  humidity. 

Thus,  although  with  a  north  wind  the  air  may  contain  a  much  small- 


THE  VARIABLE  WINDS. 


313 


er  proportion  of  vapor  of  water  than  when  the  wind  is  south,  it  is  far 
more  humid,  because  of  its  low  temperature.  The  seasons  again  modify 
this  general  rule.  The  following  is  the  influence  of  the  wind  for  each 
season,  complete  saturation  being  represented  by  1000 : 

RELATIVE  HUMIDITY  ACCOEDING  TO  THE  DIRECTION  OF  THE  WINDS 
DURING  THE  FOUR  SEASONS. 


Winds. 

Winter. 

Spring. 

Summer. 

Autumn. 

N 

895 

750 

676 

787 

N.E  

912 

723 

674 

826 

E  

926 

669 

613 

757 

S  E 

855 

714 

663 

792 

S  . 

830 

703 

674 

762 

s.w  

819 

703 

699 

786 

w  

809 

717 

714 

806 

N.W  

832 

734 

688 

327 

The  contrast  here  shown  between  winter  and  summer  is  striking. 
Although  in  these  two  seasons  the  proportion  of  vapor  is  less  with  an 
easterly  than  with  a  westerly  wind,  nevertheless  the  low  temperature 
of  these  winds  in  winter  re-establishes  the  equilibrium,  and  in  this  sea- 
son the  east  wind  is  the  most  humid  and  the  west  the  driest.  In  sum- 
mer it  is  just  the  contrary;  it  is  when  either  of  these  winds  begins  to 
blow  that  the  contrast  is  the  most  striking.  If,  for  instance,  in  winter 
the  westerly  vvinds  have  prevailed  for  some  time,  the  sky  being  clear, 
and  there  suddenly  springs  up  an  east  or  a  north-east  breeze,  then  the 
sky  becomes  cloudy,  and  the  lower  regions  of  the  atmosphere  become 
filled  with  mist.  But  if  the  wind  continues  to  blow,  then  the  sky  be- 
comes clear  again,  although  the  air  remains  moist.  If  the  reverse  takes 
place — that  is  to  say,  if  the  sky  is  overcast,  the  wind  being  in  the  east, 
and  if  it  suddenly  veers  round  to  the  south — the  sky  becomes  clear  and 
the  atmosphere  dry,  the  reason  being  that  the  heated  air  dissolves  the 
vapor  of  water  and  becomes  further  removed  from  the  point  of  satura- 
tion. It  is  only  when  this  wind  has  prevailed  for  several  days  and 
collected  a  large  quantity  of  vapor  that  the  atmosphere  again  becomes 
humid. 

We  will  now  consider  the  force  and  velocity  of  the  wind.  It  is  at 
times  very  gentle,  and  at  others  extremely  powerful.  No  other  ele- 
ment is  so  capricious  and  so  changeable ;  none  so  capable  of  soft  ca- 
resses or  of  wild  rage.  The  scale  of  its  variations  is  so  extensive  that 
it  is  difficult  to  give  a  very  exact  account  of  its  range,  from  the  breeze 
which  scarcely  raises  a  ripple  on  the  surface  of  a  lake  to  the  hurri- 
cane which  uproots  trees  and  throws  down  buildings.  The  following 


314 


THE  ATMOSPHERE. 


table  will  give  an  idea  of  the  different  degrees  of  velocity  which  it  ac- 
quires : 

TABLE  OF  THE  VELOCITY  OF  WIND. 


Velocity  per 
Second  near- 
ly in  Feet. 

Approximate 
Velocity  per 
Hour  in  Mies. 

1£ 

1 

3 

2 

6 

4 

M"gd*    f662^  d  

17 

.      12 

25 

17 

32 

22 

50 

35 

65 

45 

75 

50 

90 

60 

117 

'      80 

Tempest                        

147 

100 

160 

110 

220 

150 

Maximum  of  the  rotation  and  of  translation  as  well  

260 

180 

It  is  not  known  to  what  degree  of  speed  masses  of  air  borne  off  by 
cyclones  may  attain,  for  it  is  in  the  upper  regions  of  the  atmosphere, 
where  there  is  but  a  feeble  resistance  to  aerial  currents,  that  the  wind 
of  the  tempest  must  be  most  rapid.  Therefore  it  is  not  enough  to  as- 
certain the  rate  of  speed  of  the  rnoleculas  of  air  near  the  level  of  the 
ground  in  order  to  form  an  idea  of  the  rapidity  at  which  the  atmos- 
pheric mass  moves  when  hurried  along  by  the  tempest.  I  have  re- 
marked in  my  aerial  travels  that  the  speed  of  air  generally  increases  in 
proportion  to  the  height.*  The  balloon  which,  during  the  siege  of 


*  [On  March  31, 1863,  the  balloon  left  the  Crystal  Palace,  Sydenham,  at  4  hrs.  16  min.  P.M., 
and  fell  at  Barking,  in  Essex,  a  point  fifteen  miles  from  the  place  of  ascent,  at  6  hrs.  30  min. 
P.M.  Leaving  out  of  the  calculation  all  motion  of  the  balloon,  excepting  the  distance  between 
the  places  of  ascent  and  descent,  its  hourly  velocity  was  seven  miles  ;  the  horizontal  movement 
of  the  air  at  Greenwich,  as  shown  by  Eobinson's  anemometer,  was  five  miles  per  hour. 

On  April  18,  1863,  the  balloon  left  the  Crystal  Palace  at  1  hr.  16  min.  P.M.,  and  descended 
at  Newhaven  at  2  hrs.  46  min.  The  distance  is  about  forty-five  miles  passed  over  in  an  hour 
and  a  half,  or  at  the  rate  of  thirty  miles  per  hour.  Kobinson's  anemometer  had  registered 
less  than  two  miles  per  hour. 

On  June  26,  1863,  the  balloon  left  Wolverton  at  1  hr.  2  min.  P.M.,  and  fell  at  Littleport  at 
2  hrs.  28  min.  P.M.  The  distance  between  these  two  places  is  sixty  miles;  the  velocity  was 
therefore  forty-two  miles  per  hour.  The  anemometer  at  Greenwich  registered  ten  miles  per 
hour. 

On  July  11,  1863,  the  balloon  left  the  Crystal  Palace  at  4  hrs.  53  min.  P.M.,  and  fell  at 
Goodwood  at  8  hrs.  50  min.  P.M.,  h'aving  traveled  seventy  miles,  or  at  the  rate  of  eighteen 
miles  per  hour.  The  anemometer  at  Greenwich  registered  less  than  two  miles  per  hour. 

On  July  21,  1863,  the  balloon  left  the  Crystal  Palace  at  4  hrs.  52  min.  P.M.,  and  fell  near 


THE  VARIABLE  WINDS.  3^5 

Paris,  traveled  from  that  city  to  Christiania  accomplished  the  distance 
(nearly  1000  miles)  in  fifteen  hours,  or  at  the  rate  of  66^  miles  per 
hour ;  and  this,  although  there  was  but  little  wind  on  the  ground.  The 
balloon  sent  up  from  Paris  at  the  coronation  of  Napoleon,  in  1804  (at 
11  P.M.),  carried  the  news  of  the  Pope's  submission  to  the  emperor  di- 
rect to  Kome,  reaching  that  city  at  seven  the  next  morning,  having 
done  the  800  miles  at  an  average  hourly  speed  of  100  miles!  These 
facts  serve  to  give  us  an  idea  of  the  speed  of  the  cyclone  at  a  certain 
height  above  the  ground,  when  even  along  the  earth,  which  is  covered 
with  points  of  resistance  to  it,  its  rapidity  is  as  much  as  100  miles  in 
the  hour,  and  upon  the  ocean  150  to  170  miles. 

As  to  the  pressure  exercised  by  the  aerial  current  which  moves  at  so 
great  a  rate,  it  is  indeed  formidable.  In  a  notice  upon  the  construction 
of  light-houses,  Fresnel  calculated  that  the  highest  wind-pressure  was 
sixty  pounds  on  a  square  foot,  but  it  is  very  probable  that  in  many 

Waltham  Abbey,  having  traveled  about  twenty-five  miles  in  fifty-three  minutes,  or  at  the  rate 
of  twenty-nine  miles  per  hour.  The  horizontal  movement  of  the.  air  by  Robinson's  anemome- 
ter was  at  the  rate  of  ten  miles  per  hour. 

On  September  29, 1864,  the  balloon  left  Wolverhampton  at  7  hrs.  43  min.,  and  fell  at  Slea- 
ford,  a  point  ninety-five  miles  from  the  place  of  ascent,  at  10  hrs.  30  min.  A.M.  During  this 
time  the  horizontal  movement  of  the  air  was  thirty-three  miles,  as  registered  at  Wrottesley 
Observatory. 

On  October  9, 1864,  the  balloon  left  the  Crystal  Palace  at  4  hrs.  29  min.  P.M.,  and  descend- 
ed at  Pirton  Grange,  a  point  thirty-five  miles  from  the  place  of  ascent,  at  6  hrs.  30  min.  P.M. 
Robinson's  anemometer  during  this  time  registered  eight  miles  at  the  Royal  Observatory, 
Greenwich,  as  the  horizontal  movement  of  the  air. 

On  January  12,  1865,  the  balloon  left  the  Royal  Arsenal,  Woolwich,  at  2  hrs.  8  min.  P.M., 
and  descended  at  Lakenheath,  a  point  seventy  miles  from  the  place  of  ascent,  at  4  hrs.  19 
min.  P.M.  At  the  Royal  Observatory,  by  Robinson's  anemometer,  during  this  time  the  mo- 
tion of  the  air  was  six  miles  only. 

On  April  6,  1865,  the  balloon  left  the  Royal  Arsenal,  Woolwich,  at  4  hrs.  8  min.  P.M.  Its 
correct  path  is  not  known,  as  it  entered  several  different  currents  of  air,  the  earth  being  invisi- 
ble, owing  to  the  mist ;  it  descended  at  Sevenoaks,  in  Kent,  at  5  hrs.  17  min.  P.M.,  a  point 
fifteen  miles  from  the  place  of  ascent.  Five  miles  was  registered  during  this  time  by  Robin- 
son's anemometer  at  the  Royal  Observatory,  Greenwich. 

On  June  13,  1865,  the  balloon  left  the  Crystal  Palace  at  7  hrs.  0  min.  P.M.,  and  descended 
at  East  Horndon,  a  point  twenty  miles  from  the  place  of  ascent,  at  8  hrs.  15  min.  P.M.  Rob- 
inson's anemometer  during  this  time  registered  seventeen  miles  at  the  Royal  Observatory, 
Greenwich. 

On  August  29,  1865,  the  balloon  left  the  Crystal  Palace  at  4  hrs.  6  min.  P.M.,  and  descend- 
ed at  Wey bridge  at  5  hrs.  30  min.  P.M.,  a  point  thirteen  miles  from  the  place  of  ascent.  Dur- 
ing this  time  fifteen  miles  was  registered  by  Robinson's  anemometer  at  the  Royal  Observatory, 
Greenwich.  — ED.  ] 


316  THE  ATMOSPHERE. 

cases  this  is  exceeded.  Leaving  out  of  the  question  the  effects  of 
strong  cyclones  in  the  tropics,  several  cases  have  occurred  in  the  tem- 
perate zones  where  the  pressure  exercised  by  the  wind  in  a  very  limited 
space  was  much  above  the  calculations  of  meteorologists.  To  cite  only 
one  instance,  the  tempest  which  occurred  on  the  27th  February,  1860, 
and  which  blew  from  the  west  in  the  plains  of  Narbonne,  was  so  violent 
as  to  blow  trains  off  the  rails  on  the  line  between  Salces  and  Kivesaltes. 
The  pressure  must  have  been  at  least  eighty  pounds  to  the  square  foot. 

It  has  been  calculated  that,  approximately,  the  mechanical  force  of 
the  wind  is  in  proportion  to  the  surface  of  the  object  exposed  to  it,  and 
in  direct  ratio  to  the  square  of  the  velocity,  and  that  for  a  velocity  of  a 
yard  per  second,  for  each  square  yard,  the  effect  produced  is  about 
a  quarter  of  a  pound.  With  strong  winds,  the  velocity  of  which  is 
twenty  yards  per  second,  there  is  a  pressure  of  ten  pounds  per  square 
foot;  when,  as  in  hurricanes,  the  speed  is  forty  yards,  the  pressure  be- 
comes quadrupled.  This  renders  it  easy  to  understand  how  trees  are 
.uprooted  and  houses  blown  down. 

The  extreme  smalln-ess  of  the  molecules  of  air  is  often  more  than  com- 
pensated by  the  rapidity  of  their  motion,  so  that  they  are  capable  of 
producing  effects  which  appear  incredible,  but  which  are  in  conformity 
with  the  laws  of  mechanics. 

To  give  a  correct  idea  of  these  effects,  I  may  anticipate  the  chapter 
upon  Cyclones,  and  cite  a  few  of  the  great  disasters  caused  by  certain 
hurricanes.  At  Guadaloupe,  on  July  25th,  1825,  solidly  -  constructed 
houses  were  demolished,  and  a  new  building  belonging  to  the  State  had 
one  wing  completely  blown  down. 

The  wind  had  imparted  such  a  rate  of  speed  to  the  tiles,  that  many  of 
them  penetrated  through  thick  doors. 

A  piece  of  deal,  thirty-nine  inches  long,  ten  inches  wide,  and  nearly 
one  inch  thick,  moved  through  the  air  so  rapidly,  that  it  went  right 
through  a  palm-tree,  eighteen  inches  in  diameter. 

A  piece  of  wood  about  eight  inches  wide,  and  four  or  five  yards  long, 
projected  by  the  wind  along  a  hard  road,  was  driven  a  yard  deep  into 
the  ground. 

A  large  iron  railing,  in  front  of  the  Governor's  Palace,  was  shattered 
to  pieces. 

Three  twenty-four  pounders  were  blown  from  one  end  of  a  battery  to 
the  other. 

In  1823,  a  hurricane,  about  half  a  mile  in  diameter,  passed  close  by 


THE  VARIABLE  WINDS.  3^7 

Calcutta,  killed  in  the  space  of  four  hours  215  persons  and  wounded 
223,  blew  down  1239  fishermen's  huts,  and  drove  a  piece  of  bamboo 
through  a  wall  five  feet  in  thickness :  the  blast  of  the  air  must  have  had 
a  force  equal  to  that  of  a  six-pounder  cannon. 

At  St.  Thomas,  in  1837,  the  fortress  which  protects  the  entrance  into 
the  harbor  was  demolished  as  if  by  bombardment.  Fragments  of  ro6k 
were  projected  from  a  depth  of  thirty  to  forty  feet,  and  hurled  on  the 
shore.  In  other  places,  strong  houses,  torn  up  from  their  foundations, 
were  swept  along  the  ground  before  the  wind.  On  the  banks  of  the 
Ganges,  the  Antilles  coast,  and  at  Charlestown,  several  vessels  were  car- 
ried from  the  sea  some  distance  inland.  In  1681,  an  Antigua  vessel  was 
carried  out  of  the  water  to  a  point  ten  feet  above  the  highest  known 
tide.  In  1825,  the  vessels  which  were  in  the  harbor  of  Basseterre  dis- 
appeared, and  one  of  the  captains,  who  had  escaped,  said  that  his  ship 
was  lifted  by  the  hurricane  out  of  the  sea,  and  was,  so  to  speak,  "  ship- 
wrecked in  the  air."  A  quantity  of  the  debris  from  Guadaloupe  was 
carried  to  Montserrat,  over  an  arm  of  the  sea  fifty  miles  wide.  In  the 
tempest  which  blew  across  the  English  Channel  on  January  llth,  1866, 
stones  weighing  from  four  to  six  hundred  pounds  were  hurled  over  the 
Breakwater  at  Cherbourg  to  a  height  of  more  than  eight  yards.  Ad- 
miral Le  Noury  states  that  the  sea  dashed  against  the  fort,  which  is  185 
feet  above  the  level  of  the  shore. 

The  only  difficulty  in  explaining  these  phenomena  is  to  discover  how 
the  air  can  attain  in  the  atmosphere  so  prodigious  a  velocity ;  for, 
granting  that  velocity,  the  most  extraordinary  chemical  action  becomes 
the  necessary  consequence.  It  is  gas  in  motion  which  drives  the  can- 
non-ball from  the  gun,  and  which  hurls  into  the  air  vast  masses  of  rock 
when  a  mine  explodes.  An  oak  plank,  nearly  an  inch  thick,  may  be 
pierced  by  a  candle  fired  out  of  a  gun ;  the  force  of  the  projectile  being 
only  due  in  this  case  to  its  velocity. 


318  THE  ATMOSPHERE. 


CHAPTEE  IY. 

RESPECTING  CERTAIN  SPECIAL  WINDS:  THE  BISE  —  THE  BORA  —  THE 
GALLEGO  — THE  MISTRAL  — THE  H  ARM  ATT  AN  —  THE  SIMOOM  — THE 
KHAMSEEN — THE  SIROCCO — THE  SOLANO. 

HAVING  considered  the  theory  and  the  action  of  the  general  winds 
(both  those  that  are  regular  and  irregular)  which  blow  over  the  surface 
of  the  globe,  we  must  now  turn  our  attention  to  special  winds  which 
characterize  certain  countries,  and  to  atmospheric  movements  which  at 
times  traverse  oceans  and  continents  with  the  rapidity  of  a  bird  of  prey, 
and  which  seem  to  form  an  exception  to  the  system  of  organized  laws 
by  which  nature  is  regulated.  Scientific  analysis  has  shown  that  these 
phenomena  are  obedient,  like  every  thing  else  in  the  universe,  to  defi- 
nite and  fixed  laws. 

In  France,  the  temperate  climate  which  we  enjoy  precludes  the  in- 
tense atmospheric  phenomena  which  occur  in  less  favored  regions. 
Among  the  winds,  properly  so  called,  which  differ  slightly  in  their  char- 
acter from  most  of  the  general  winds,  may  first  be  cited  the  bise,  or 
north  wind,  which  is  very  cold,  and  occasionally  very  violent.  In  the 
east  of  France  it  is  much  dreaded,  for  it  comes  nearly  in  a  straight  line 
from  the  North  Sea ;  and  having  traversed  Holland  and  Belgium  when 
those  countries  are  covered  with  snow,  it  becomes  even  colder  during 
its  passage.  At  Istria  and  in  Dalmatia  the  bise  is  known  as  the  bora, 
and  it  is  so  strong  that  it  sometimes  blows  over  a  horse  and  cart.  In 
Spain,  this  same  north  wind — which  is  sometimes  a  north-east  wind  in 
that  region — is  designated  the  gallego. 

In  the  south  of  France,  the  cold  and  violent  south-west  wind  which 
has  passed  over  the  snows  of  the  Alps  and  the  Pyrenees,  and  which  is 
known  as  the  mistral,  deserves  particular  notice. 

Its  cause  was  long  unknown.  It  was  attributed  to  a  sudden  coldness 
of  the  wind  that  passed  over  the  Alps  and  the  Pyrenees.  M.  Marie- 
Davy,  in  several  notes  published  in  the  Bulletin  de  F  Observatoire  in  June, 
1864,  proved  that  the  cause  of  this  wind  is  not  local,  and  that  the  move- 
ments which  give  rise  to  it  pass  eastward  like  whirlwinds.  Kaemtz,  in 
a  communication  to  the  Institute  in  July,  1865,  shows,  by  means  of  a 


RESPECTING  CERTAIN  SPECIAL  WINDS.  319 

list  of  barometrical  pressures  in  France,  Spain,  and  Italy,  before,  during, 
and  after  the  passage  of  the  mistral,  that  it  is  a  regular  tempest,  coming 
from  a  great  distance,  and  that  it  is  not  due  to  a  sudden  fall  in  the  tem- 
perature of  the  wind  while  passing  over  the  mountains.  • 

It  is  remarkable  that,  in  proportion  as  meteorology  advances,  we  learn 
not  to  look  for  the  causes  of  most  phenomena  in  the  localities  where 
they  occur,  but  to  general  preponderating  causes  to  which  the  local  cir- 
cumstances are  subordinate. 

Whenever  the  mistral  blows,  there  is  an  excess  of  atmospheric  press- 
ure to  the  west  of  the  Gulf  of  Lyons.  Whatever  may  be  the  origin  of 
this  pressure,  it  always  is  an  accompaniment  of  the  mistral. 

For  the  mistral  to  occur,  no  matter  in  what  season,  there  must  always 
be  a  combination  of  circumstances  which  are  always  identically  alike. 
Whether  there  be  fine  or  bad  weather  in  the  south-west  of  Europe, 
there  is  always  an  excess  of  pressure  to  the  west  of  the  Cevennes. 

The  violence  of  the  wind  is  due  to  the  form  of  the  Pyrenean  isthmus. 
As  soon  as  the  general  direction  of  the  atmospheric  movement  veers 
slightly  from  west  to  north,  the  central  plateau  and  the  main  body  of 
the  Alps  cause  an  inclination  of  the  current  toward  the  Grulf  of  Lyons. 
This  current,  compressed  between  the  Alps  and  the  Pyrenees  in  the  di- 
rection of  length,  and  by  the  Cevennes  in  a  vertical  direction,  consti- 
tutes a  rapid  upon  the  coast  of  Languedoc,  and  this  is  one  of  the  causes 
of  the  excess  of  pressure  upon  the  north-east  slope  of  the  Cevennes,  and 
of  the  diminution  in  pressure  upon  the  Mediterranean,  where  the  wind 
maintains  a  velocity  no  longer  commensurate  with  the  width  of  the 
channel.  Hence  also  arises  the  violence  of  the  north  wind  in  the  valley 
of  the  Ehone  between  the  spurs  of  the  Alps  and  those  of  the  central 
plateau. 

The  mistral  is  the  driest  of  the  winds  in  these  regions,  because  it  has 
been  rendered  dry  in  its  passage  over  the  Cevennes.  It  is,  indeed,  plu- 
vious or  moist  upon  the  north-western  slope  of  those  mountains.  The 
winds  from  the  east  or  south  regions  bring  rain  with  them,  because  they 
are  sea- winds  upon  the  coasts  and  upon  the  south-western  slope  of  the 
Cevennes ;  they  are  dry  upon  the  opposite  slope. 

The  high  temperature  of  the  interior  of  Africa  is  the  cause  of  the  ex- 
traordinary winds  which  are  met  with  on  the  coasts  of  Guinea  and  Bar- 
bary,  in  Egypt,  Arabia,  Syria,  the  steppes  of  Southern  Eussia,  and  even 
in  Italy.  These  winds,  the  names  of  which  are  harmattan,  simoom,  and 
khamseen,  have  unusual  accompaniments,  some  details  of  which  it  may 


320  THE  ATMOSPHERE. 

be  interesting  to  mention.     They  are  remarkably  hot  and  dry,  and  are 
attended  by  whirlwinds  of  dust. 

The  name  harmattan  is  given  to  a  wind  which  blows  three  or  four 
times  each  season  from  the  interior  of  Africa  toward  the  Atlantic,  in 
the  space  comprised  between  Cape  Verd  (lat.  14°  44'  N.)  and  Cape  Lo- 
pez, on  the  African  coast  near  the  equator.  The  harmattan  is  generally 
noticed  in  the  months  of  December,  January,  and  February.  Its  direc- 
tion is  from  E.S.B.  to  N.N.E.,  and  its  ordinary  duration  is  one  or  two 
days — sometimes  five  or  six.  This  wind  is  only  moderately  strong. 
A  peculiar  kind  of  mist,  so  thick  as  to  shut  out  all  but  a  few  red  rays  of 
the  sun  at  noon,  always  rises  when  the  harmattan  begins  to  blow.  The 
particles  of  which  this  mist  is  composed  alighting  upon  turf,  the  leaves 
of  trees,  and  the  skin  of  the  negroes,  make  every  thing  white.  Their 
nature  is  not  known,  and  all  that  has  been  ascertained  respecting  them 
is,  that  they  are  carried  but  a  very  little  way  out  to  sea :  at  a  league 
from  the  shore,  for  instance,  the  mist  is  very  slight,  and  at  three  leagues 
there  is  no  trace  of  it,  though  the  harmattan  may  be  blowing  with  its 
full  force. 

The  extreme  dryness  of  the  harmattan  is  one  of  its  most  marked  char- 
acteristics. If  it  continues  any  length  of  time,  the  branches  of  the  or- 
ange and  lemon  trees  become  parched  and  begin  to  die;  the  covers  of 
books,  not  even  excepting  those  which  are  wrapped  up  in  linen  and  in- 
closed in  a  case,  become  bent  as  if  they  had  been  laid  before  a  fire.  The 
panels  of  doors  and  windows,  the  furniture  of  rooms,  crack,  and  often 
snap.  The  effect  of  this  wind  upon  the  human  body  is  not  less  pro- 
nounced. The  eyes  and  the  lips  dry  up  and  smart.  If  the  continuance 
of  the  harmattan  be  for  four  or  five  consecutive  days,  the  cuticle  of  the 
hands  and  the  face  begins  to  peel,  and  it  is  necessary  to  anoint  the  body 
with  grease. 

All  this  would  lead  one  to  suppose  that  the  harmattan  must  be  very 
unhealthy ;  but,  so  far  from  this  being  the  case,  it  is  the  opposite.  In- 
termittent fever,  for  instance,  is  radically  cured  by  the  first  breath  of 
the  harmattan.  Persons  who  have  become  weakened  by  the  excessive 
blood-letting  practiced  in  those  countries  at  once  recover  their  strength ; 
remittent  and  epidemic  fevers  also  disappear  as  if  by  enchantment.  So 
salutary,  in  fact,  is  the  influence  of  this  wind  while  it  lasts,  that  it  is 
said  to  be  impossible  to  communicate  infection  even  artificially,  for  it 
appears  that  vaccine  virus  will  not  act  during  its  continuance. 

Its  asserted  poisonous  properties  are  therefore  pure  invention,  and 


RESPECTING   CERTAIN  SPECIAL  WINDS.  321 

may  possibly  have  been  circulated  by  the  Arabs  to  deter  travelers  from 
penetrating  into  what  they  consider  their  kingdom. 

Kaemtz  says:  "At  every  epoch  the  Arab  of  the  desert,  poor  and  of 
nomad  habits,  has  detested  the  inhabitant  of  towns  who  leads  a  steady 
and  orderly  life.  Thus,  when  the  merchant  is  compelled  to  cross  the 

desert,  the  Bedouin  exacts  an  enormous  price  for  protecting  him 

In  the  eyes  of  the  inhabitant  of  a  town  the  desert  was  the  theatre  of 
scenes  horrible  beyond  description.  All  the  marvelous  stories  of  ad- 
venture found  a  ready  audience,  just  as  in  our  days  the  Turks  have  the 
most  grotesque  and  unreal  ideas  concerning  Europe.  The  dwellers  in 
the  desert  took  care  not  to  destroy  these  fancies,  but  rather  to  confirm 
them,  and  the  few  merchants  who  knew  the  exact  truth  kept  it  to  them- 
selves in  order  to  maintain  a  monopoly  of  commerce.  It  is  in  this  way 
that  visionary  ideas  maintain  their  sway." 

The  Arab  writers  teem  with  falsehoods  concerning  the  desert,  and  the 
European  travelers  have  outrivaled  them.  The  Mussulman  believes 
he  is  acting  meritoriously  when  he  deceives  the  unbelievers  and  keeps 
them  away  from  the  desert.  L.  Burckhardt,  of  Bale,  was  the  first  who 
supplied  reliable  information  concerning  the  phenomena  of  the  desert, 
and  especially  touching  the  winds  which  prevail  there.  He  thus  re- 
duced to  their  true  value  the  fabulous  stories  of  his  predecessors,  Beau- 
champ,  Bruce,  and  Niebuhr.  t  \ 

Burckhardt  relates  that  this  wind  of  the  desert  surprised  him  between 
Siout  and  Esne.  He  says,  "When  it  rose  I  was  alone,  mounted  upon 
my  dromedary,  and  far  away  from  any  houses  or  trees.  I  endeavored 
to  protect  my  face  by  covering  it  with  a  handkerchief.  In  the  mean 
while,  the  dromedary,  into  whose  eyes  the  sand  was  driving,  became 
alarmed,  and  began  to  gallop,  causing  me  to  fall  off.  I  remained  flat 
upon  the  earth,  for  I  could  not  see  ten  yards  in  front  of  me,  and  I  cov- 
ered myself  with  my  clothes  as  well  as  I  could  until  the  wind  became 
less  violent.  I  then  went  in  search  of  the  dromedary,  which  I  found 
some  distance  off,  lying  with  his  head  against  a  bush  to  protect  it  from 
the  sand."  Malcolm  and  Morier,  who  crossed  the  Persian  deserts,  Ker- 
Porter,  who  visited  that  which  lies  to  the  east  of  the  Euphrates,  agree 
with  Burckhardt  that  when  they  were  exposed  to  the  simoom  they  felt 
no  ill  effects  from  it  beyond  the  momentary  inconvenience. 

"  It  is  not  only  in  the  sandy  deserts  of  Africa  and  Asia  that  the  hot 
winds  are  to  be  dreaded,  but  in  nearly  all  continental  countries  near 
the  tropics.  In  India  they  are  known  as  the 'devil  winds.'  They  fre- 

21 


322  THE  ATMOSPHERE. 

quently  occur  during  the  dry  season,  and  scatter  terror  and  desolation 
through  country  and  town.  Without  being  absolutely  poisonous,  it 
may  be  admitted  that  winds  whose  speed  is  so  formidable,  laden  with 
grains  of  sand,  and  the  temperature  of  which  is  as  much  as  104°,  may 
exercise  an  unhealthy  influence  upon  the  regions  through  which  they 
pass,  and  be  especially  dangerous  for  Europeans  who  do  not  know  how 
to  protect  themselves." 

About  the  time  of  the  equinox  the  tempests  in  the  desert  become  ter- 
rible. Every  one  has  heard  of  the  burning  wind,  the  simoom — a  wind 
which,  in  Arabia,  means  poison.  This  formidable  wind  blows  also  in 
Egypt,  where  it  is  called  khamseen  (fifty),  because  it  lasts  that  number 
of  days,  five-and-twenty  before,  and  five-and-twenty  after  the  spring 
equinox. 

The  simoom  is  preceded  by  a  black  spot  which  rises  in  the  horizon. 
This  spot  grows  rapidly  larger.  A  murky  veil  obscures  the  sky,  gusts 
of  sand  darken  the  sun  and  dry  up  all  verdure.  As  soon  as  it  begins 
to  blow,  birds  fly  off  affrighted ;  the  dromedary  seeks  a  bush  to  pro- 
tect him  from  the  sand ;  the  Arab  covers  his  face,  rubs  his  body  with 
grease  or  wet  mud,  and  lies  on  the  ground  or  hides  himself  behind  a 
tree  until  it  is  over.  The  simoom  is  the  most  dangerous  of  the  acci- 
dents to  which  a  caravan  crossing  the  desert  can  be  exposed,  and  to  it 
is  attributed  the  destruction  of  the  50,000  men  sent  by  Cambyses  to 
destroy  the  Temple  of  Jupiter  Ammon. 

In  1805  the  simoom  buried  in  the  sand  a  whole  caravan,  causing  the 
death  of  2000  men  and  1800  camels.  More  than  once  French  generals 
have  feared  that  columns  of  troops  which  they  have  been  obliged  to 
send  into  the  desert  have  been  overtaken  and  destroyed  by  the  simoom. 

The  impalpable  dust  which  the  air  carries  along  in  thick  clouds 
enters  the  nostrils,  the  eyes,  the  mouth,  and  the  lungs,  and  causes  as- 
phyxia. When  it  does  not  absolutely  kill,  the  rapid  evaporation  from 
the  surface  of  the  body  dries  up  the  skin,  inflames  the  throat,  makes 
the  breathing  rapid,  and  produces  violent  thirst.  The  terrible  blast  of 
the  simoom  dries  up  the  sap  of  trees  in  its  passage,  and  causes  the 
water  contained  in  the  skins  carried  by  the  camels  to  evaporate.  The 
caravan  is  then  a  prey  to  horrible  thirst,  which  sets  the  blood  on  fire, 
and  the  route  which  they  follow  is  strewed  with  the  whitened  bones  of 
men  and  animals  who  perish  for  want  of  water. 

Thomas  William  Atkinson  was  a  witness,  in  1850,  of  the  fierce  hur- 
ricanes which  swoop  down  upon  the  steppes  of  Mongolia.  He  says, 


RESPECTING   CERTAIN  SPECIAL   WINDS.  325 

"A  solemn  silence  prevails  in  these  vast  and  arid  plains,  which  are  de- 
serted alike  by  men,  quadrupeds,  and  birds.  People  talk  of  the  soli- 
tude of  forests.  I  have  often  ridden  through  their  sombre  alleys  for 
days  together :  the  soughing  of  the  wind,  the  cracking  of  the  branches, 
and  the  murmur  of  the  leaves  are  to  be  heard  there ;  sometimes,  too, 
the  crash  of  one  of  the  giants  of  the  forest  as  it  falls  to  the  ground 
wakes  the  distant  echoes  and  startles  from  their  lairs  the  tenants  of  the 
glades,  making  the  birds  utter  a  cry  of  terror.  This  is  not  solitude : 
trees  and  leaves  have  a  language  which  man  recognizes  from  a  great 
distance;  but  in  these  arid  deserts  there  is  no  sound  to  break  the 
death-like  silence  which  prevails. 

"The  sand  was  raised  into  circular  terraces,  some  from  fifteen  to 
twenty  feet  high,  and  they  extended  as  far  as  the  eye  could  reach  into 
the  desert.  Seen  from  one  of  the  knolls  they  presented  the  singular 
appearance  of  an  immense  necropolis,  over  which  were  dotted  countless 
tumuli. 

"  While  I  was  taking  a  sketch  of  this,  I  was  witness  of  the  formation 
of  a  hurricane  above  the  level  of  the  water.  It  was  coming  from  the 
north  direct  upon  us.  The  Cossacks  and  Tchuck-a-boi  went  to  place 
their  horses  in  security  behind  the  reeds  and  bushes,  leaving  two  of 
their  companions  with  me.  The  tempest  came  on  at  a  fearful  rate, 
driving  enormous  waves  into  the  air,  and  striking  down  all  vegetation 
in  its  path.  A  long  white  wave  came  moving  along  the  lake,  and 
when  it  was  within  half  a  verst  its  roar  became  audible.  The  men 
begged  me  to  move  away,  and  we  rejoined  the  rest  of  the  troop  behind 
the  bushes.  I  had  scarcely  reached  there  when  the  hurricane  burst 
forth,  bending  the  bushes  to  the  ground.  "When  it  reached  the  sand  it 
began  to  revolve  in  a  circle,  lifting  whole  mounds  of  sand  into  the  air, 
and  causing  others  to  spring  up  where  there  had  been  none  previous  to 
the  storm.  This  tempest  lasted  a  quarter  of  an  hour,  when  the  atmos- 
phere became  as  calm  as  it  had  been  before. 

"  It  is  very  dangerous  to  be  overtaken  in  the  plain  by  one  of  these 
typhoons.  I  have  since  seen  them  swoop  down  from  the  mountains  or 
rise  from  the  hollow  of  some  deep  ravine,  in  the  shape  of  a  black  and 
compact  mass,  with  a  diameter  of  as  much  as  1000  yards  or  more,  and 
rushing  along  the  steppe  with  the  speed  of  a  race-horse.  All  animals, 
whether  tame  or  savage,  take  flight  before  it,  for  once  enveloped  in  its 
sphere  of  action  they  must  infallibly  perish.  The  wild  horses  gallop 
off  in  terror  before  the  storm  which  pursues  them." 


326  THE  ATMOSPHERE. 

In  Europe  we  have  the  sirocco  (Italy),  and  the  solano  (Spain),  both 
of  which  have  a  very  enervating  effect  upon  those  exposed  to  them. 

Brydone,  who  was  at  Palermo  on  July  8th,  1770,  during  a  sirocco, 
writes,  "  I  opened  my  door  at  eight  in  the  morning  without  suspecting 
there  was  any  change  in  the  temperature,  when  all  at  once  I  felt  a 
burning  impression  upon  my  face  like  the  air  from  a  hot  oven.  I 
closed  my  door,  exclaiming  to  Fullarton  that  all  the  atmosphere  was  on 
fire."  At  this  moment  the  thermometer,  in  the  open  air,  marked  111°. 

An  army  surgeon  who  accompanied  the  French  troops  in  a  march 
between  Oran  and  Tlemcen,  in  the  desert,  gives  the  following  account 
of  a  sirocco:  "It  was  toward  the  end  of  July,  1846.  Several  soldiers 
had  succumbed  to  the  heat.  The  sirocco  assailed  our  little  column. 
Under  the  influence  of  this  dry,  heavy,  and  enervating  air,  the  breath- 
ing became  difficult;  the  lips  and  the  nostrils,  cracked  by  the  burning 
dust  driven  up  by  the  wind,  were  both  dry  and  painful,  and  the  throat, 
as  it  were,  became  contracted.  The  face  was  burned  by  gusts  of  heat, 
sometimes  followed  by  tremor,  and  a  fainting  away  which  resembled 
syncope.  The  perspiration  ran  off  in  streams,  and  the  water,  which 
was  eagerly  swallowed,  did  not  quench  the  thirst,  but  increased  the 
stomachic  pains  and  the  difficulty  of  breathing.  To  walk  would  have 
been  impossible ;  we  felt  half  suffocated  under  cover  of  the  tents,  and 

in  the  open  air  the  burning  breeze  caused  a  choking  sensation 

But  for  the  water,  our  column  must  have  perished." 


THE  POWER  OF  THE  AIR. 


327 


CHAPTER  V. 

THE    POWER   OF    THE    AIR:     THE    HURRICANE  —  THE    CYCLONE  —  THE 
TEMPEST. 

THE  two  great  general  currents  which  have  been  adverted  to,  the 
one  moving  from  the  equator  to  the  poles,  and  the  other  from  the  poles 
to  the  equator,  come  into  collision  with  each  other  in  the  equatorial 
zone.  Various  causes  counterbalance  the  periodical  action  of  the  solar 
rays,  and  place  obstacles  in  the  way  of  the  ordinary  progress  of  air. 
The  diversity  in  the  temperature  of  continents  and  seas  causes  a  varia- 
tion in  the  normal  direction  and  intensity  of  the  currents.  The  state 
of  the  sky  in  the  tropics,  according  as  it  remains  clear  or  cloudy  for 
any  length  of  time  together,  condenses  the  heat  as  in  a  focus  of  absorp- 
tion, or  disseminates  it  over  vast  tracts  of  country.  The  undulations 
of  the  soil,  the  high  chains  of  mountains  and  their  temperature,  the  less 
lofty  plateaux,  and  even  the  valleys  themselves,  cause  in  one  place  the 
storing  up  and  the  repose  of  large  masses  of  air,  in  another  their  dis- 
tribution in  different  directions,  while  in  other  cases  this  same  uneven- 
ness  of  the  ground  forces  the  currents  back  right  and  left,  causing  them 
to  eddy  like  the  waters  of  a  stream,  or  to  rush  furiously  past  the  ob- 
stacles in  their  way.  The  blasts  of  the  air  as  they  meet,  either  join 
forces  or  oppose  each  other,  increasing  or  destroying  their  mutual 
power.  It  is  in  this  way  that  strong  winds,  hurricanes,  and  tempests 
arise.  These  atmospheric  contentions,  which  sometimes  attain  gigantic 
proportions,  create  a  great  disturbance  in  the  course  of  nature.  They 
have  been  studied  by  sailors  and  meteorologists,  and  the  principal  laws 
which  seem  to  regulate  them  have  been  ascertained.  Eedfield  and 
Reid,  Professor  Dove,  of  Berlin,  and  Admiral  Fitzroy,  have,  after  great 
labor,  succeeded  in  forming  a  theory  of  the  tempests  which  explains 
the  most  violent  of  the  movements  in  the  atmosphere,  and  their  re- 
searches will  be  useful  in  considering  this  subject  here. 

One  of  the  chief  observations  made  is,  that  hurricanes  do  not  proceed 
in  straight  lines,  but  follow  a  curve,  turning  horizontally  upon  their 
own  axes  by  a  rapid  rotatory  movement. 

This  characteristic  movement  of  horizontal  rotation  has  earned  for 


328  THE  ATMOSPHERE. 

these  gigantic  whirlwinds  the  name  of  cyclone  (icuicXoc,  circle).  They 
are  the  general  hurricanes,  which  are  not  local  tempests  resulting  from 
the  deviation  of  the  wind,  owing  to  the  configuration  of  the  soil  or  the 
meeting  of  several  ordinary  currents,  but  extend  over  several  hundred 
square  leagues,  and  travel  a  distance  of  many  thousand  miles. 

The  cyclones  are  vast  whirlwinds,  of  various  size  in  diameter,  in 
which  the  force  of  the  wind  increases  from  all  the  points  of  the  circum- 
ference to  the  centre,  where  a  calm  prevails.  In  this  centre,  however, 
the  sea  remains  rough.  There  is  no  cloud  in  this  calm  region ;  the  sun 
shines  brightly,  the  stars  appear,  and  fine  weather  seems  to  hav^e  return- 
ed, when  in  reality  it  is  surrounded  on  all  sides  by  a  vast  belt  o£  fierce 
hurricanes. 

All  around  this  central  calm  the  rotatory  movement  is  of  the  same 
force,  and  this  force  is  the  greatest. '  Consequently,  on  passing  to  this 
central  region,  a  ship  passes  from  a  violent  tempest  into  a  complete 
calm,  and,  on  crossing  this  calm  space,  passes  on  the  opposite  side  into 
a  violent  tempest  again.  But  in  this  latter  case  the  hurricane  blows  in 
the  opposite  direction  to  what  it  did  before  entering  the  calm,  since  the 
movement  of  cyclones  is  circular.  ^ 

The  first  central  zone,  which  constitutes  in  reality  a  hurricane,  and 
during  the  passage  of  which  occur  all  the  disasters,  -is  generally  from 
100  to  120  leagues  in  diameter,  whatever  may  be  the  extreme  limits 
which  the  phenomenon  reaches,  for  its  power  is  not  in  proportion  to  its 
extent. 

The  rotatory  speed  of  the  hurricanes  varies  very  much :  it  is  that 
which  constitutes  chiefly  the  violence  of  the  whirlwind,  and  which 
causes  it  to  be,  in  regard  to  the  places  against  which  it  blows  and  the 
vessels  it  assails,  either  a  hurricane,  a  gust  of  wind,  or  a  simple  gale.  In 
violent  storms,  it  is  estimated  that  the  moleculse  of  air  turn  around  its 
centre  with  a  rotatory  speed  of  sixty  leagues  an  hour — a  rapidity  which 
explains  the  ravages  and  disasters  produced  by  the  passage  of  this  ter- 
rible wind. 

The  cyclone  generally  begins  between  the  latitudes  of  5°  and  10°. 
It  moves,  in  our  hemisphere,  in  a  north-westerly  direction,  continuing 
thus  until  it  reaches  a  particular  latitude,  when  it.  turns  toward  the 
north-east,  and  thus  forms  a  parabola,  the  two  branches  of  which  di- 
verge farther  and  farther. 

The  difference  in  the  density  of  the  different  atmospheric  strata 
which  are  encountered  in  its  passage,  the  rotatory  movement  itself. 


THE  POWER  OF  THE- AIR.  32Q 

must  impart  an  oscillating  movement  to  the  cyclone,  so  that,  instead 
of  describing  a  regular  parabola,  the  course  of  the  cyclone  is  rather 
spiral,  infolding  itself  around  the  parabola.  Ships  that  happen  to  be 
in  the  midst  of  it  are  exposed  to  its  oscillating  action:  hence  those 
terrible  gales  which  are  succeeded  by  a  more  or  less  complete  calm ; 
hence  those  dramatic  situations  in  which  the  ship  in  distress  sees  the 
wind  veer  round  to  all  the  points  of  the  compass  in  a  short  space  of 
time. 

The  sudden  and  dangerous  variations  of  the  wind,  which  were  for- 
merly considered  as  essential  to  hurricanes,  typhoons,  tornadoes,  etc., 
can  not,  and  in  fact  do  not,  occur  save  in  the  immediate  path  of  the 
centre  of  the  cyclone.  The  cyclone  contains  in  itself  the  germ  of  its 
own  early  destruction :  in  proportion  as  it  advances  it  is  approaching 
nearer  to  regions  which  are  colder  than  those  whence  it  started ;  the 
vapor  which  it  contains  becomes  condensed  into  torrential  rain;  the 
electricity  issues  from  it  in  large  currents;  the  equilibrium  which  ex- 
isted becomes  destroyed ;  and  the  centrifugal  force,  being  no  longer 
counterbalanced,  permits  it  to  extend  to  an  immense  size.  It  then  loses 
in  force  what  it  gains  in  volume ;  at  its  starting-point  it  does  not  meas- 
ure more  than  a  few  leagues,  but  when,  having  lost  its  equilibrium,  it 
collapses — as  generally  occurs  between  the  latitudes  of  40°  and  45° — it 
extends  over  hundreds  of  miles. 

The  more  rapid  the  escape  of  electricity  the  quicker  will  the  meteor 
collapse:  thus  it  sometimes  happens  that  a  cyclone  terminates  its  course 
before  reaching. these  high  latitudes,  and  without  describing  the  second 
branch  of  its  parabola,  which  therefore  remains  incomplete. 

Between  latitudes  5°  and  10°,  and  longitudes  45°  and  60°,  when  a 
cyclone  is  near  its  starting-point,  it  has  been  ascertained  that  the  rate 
of  revolution  is  inconsiderable,  varying  from  one  to  five  miles  an  hour, 
increasing  as  the  hurricane  advances  westward. 

In  latitudes  35°  to  45°,  and  in  longitudes  from  50°  to  30°,  the  rate 
of  revolution  varies  from  six  to  twelve  miles  an  hour.  In  the  higher 
latitudes  it  is  greater,  and  has  been  known  to  be  as  much  as  twenty 
miles  per  hour. 

The  greatest  rate  of  revolution  ever  registered  is  that  of  a  cyclone 
which  reached  the  Banks  of  Newfoundland  from  the  Antilles  in  August, 
1853,  when  the  speed  was  thirty-one  miles  per  hour.  This  velocity 
gradually  increased  to  ninety ;  and  without  affecting  the  speed  of  rota- 
tion, which  was  sixty  leagues  an  hour.  Thus  the  wind  is  capable  of 


330  THE  ATMOSPHERE. 

traveling  along  the  surface  of  the  sea  at  a  speed  of  seventy-five  leagues 
an  hour,  perhaps  more. 

The  origin  of  cyclones,  so  far  as  can  be  judged  by  the  comparisons 
that  have  been  made,  is  probably  due  to  the  encounter  of  two  currents 
of  air  moving  in  opposite  directions.  The  place  of  meeting  forms  a 
neutral  point,  where  the  air  receives  a  rotatory  movement  from  the  col- 
lision of  the  two  currents.  It  is  like  an  eddy  in  a  stream,  and  a  mo- 
ment's reflection  will  enable  the  reader  to  form  an  idea  of  it. 

These  immense  whirlwinds  come  into  existence  to  the  south  as  well 
as  to  the  north  of  the  equator.  The  astronomer  Poey,  Director  of  the 
Observatory  at  Havana,  has  ascertained,  by  a  laborious  research  into 
the  hurricanes  which  have  raged  in  the  West  Indies  since  the  discov- 
ery of  America  (1493)  to  the  present  day,  that,  out  of  365  grand  cy- 
clones, more  than  two-thirds  have  occurred  between  the  months  of  Au- 
gust and  October,  that  is,  during  the  period  when  the  heated  shores  of 
South  America  are  beginning  to  attract  toward  them  the  colder  and 
denser  air  of  North  America.  In  the  Indian  Ocean  cyclones  are  most 
frequent  when  the  change  occurs  in  the  direction  of  the  monsoons  and 
at  the  end  of  summer.  In  the  list  of  hurricanes  in  the  southern  hemi- 
sphere, drawn  up  by  Piddington  and  completed  by  Bridet,  there  is  not 
a  single  mention  of  a  hurricane  in  the  months  of  July  or  August; 
more  than  three-fifths  took  place  in  the  first  three  months  of  the  year. 
It  is  at  the  epoch  of  change  of  seasons  that  the  powerful  masses  of  air, 
loaded  with  electricity,  enter  into  a  struggle  for  the  mastery,  and  give 
rise,  by  their  meeting,  to  those  great  eddies  which  develop  themselves 
in  a  spiral  shape  over  sea  and  land.  At  the  same  time,  the  whirlwind 
never  extends  very  high  into  the  aerial  ocean.  According  to  Bridet, 
the  average  height  of  hurricanes  in  the  Indian  Ocean  is  less  than  10,000 
feet;  Eedfield  puts  it  at  no  more  than  6000.  As  a  rule,  the  stratum  of 
air  which  revolves  in  this  way  is  not  nearly  so  thick ;  sometimes  it  is 
so  shallow  that  the  crew  of  a  vessel  which  is  exposed  to  a  cyclone  see 
above  their  heads  a  clear  sky.  Above  the  cyclone  the  storm-winds 
follow  their  regular  course. 

The  analysis  of  cyclones  is  especially  due  to  Eedfield.  America  is  a 
country  peculiarly  well  adapted  for  observing  these  phenomena,  as  the 
hurricanes  which  run  along  the  shores  of  the  United  States  pass,  dur- 
ing their  progress  through  the  tropics,  over  the  West  India  Islands, 
where  their  remarkable  character  has  earned  them  the  appellation  of 
"  West  India  hurricanes." 


THE  POWER  OF  THE  AIR.  33  ^ 

In  regard  to  the  cyclones  which  occur  in  Central  Europe,  it  is  rarely 
possible  to  ascertain  through  what  part  of  the  tropics  they  have  passed, 
and  this  is  a  sufficient  proof  that  the  wider  the  extent  of  our  observa- 
tions, the  less  likely  shall  we  be  to  form  incorrect  ideas  of  these  phe- 
nomena of  nature. 

The  meteorologist  Dove  proved,  in  his  work  upon  the  "Law  of  Tem- 
pests" (Paris  edition,  p.  173),  that  a  cyclone  movement  occurs  whenever 
any  obstacle  stands  in  the  way  of  the  regular  change  in  the  direction 
of  the  wind  (which  is  due  to  the  rotation  of  the  earth),  and  therefore 
prevents  the  regular  rotation  of  the  vane  to  some  given  point.  He 
says: 

"  The  hurricanes  in  the  West  Indies  generally  commence  at  the  in- 
ner limit  of  the  zone  of  trade-winds,  in  the  region  of  calms,  where  the 
air  rises  and  becomes  disseminated  in  the  upper  strata  of  the  atmos- 
phere, and  in  a  direction  contrary  to  that  of  the  trade-wind.  This 
renders  it  probable  that  the  primary  cause  of  cyclones  is  the  intrusion 
of  a  part  of  this  upper  current  into  that  below. 

"  Let  us  also  imagine  that  the  air  which  rises  over  Asia  and  Africa 
flows  laterally  into  the  upper  strata  of  the  atmosphere — a  fact  which  is 
made  evident  by  the  sand  which  falls  in  the  North  Atlantic,  and  which 
rises  to  a  great  height,  for  on  the  Peak  of  Teneriffe  the  sun  is  some- 
times obscured  by  it.  A  similar  current  must  have  a  tendency  to  op- 
pose the  free  passage  of  the  upper  anti-current  of  the  trade-winds,  and 
must  force  it  back  into  the  lower  current,  or  the  direct  trade-wind. 
The  point  at  which  this  intrusion  takes  place  must  advance  with  as 
great  rapidity  as  the  oblique  upper  current  which  causes  it.  The  in- 
terposition of  a  current  traveling  from  B.  to  W.  with  another  traveling 
from  S.W.  to  N.E.  must  necessarily  create  a  rotatory  movement  in  a 
direction  the  opposite  to  that  followed  by  the  hands  of  a  watch.  Ac- 
cording to  that,  the  cyclone,  which  advances  from  S.W.  toward  N.E. 
in  the  lower  trade-winds,  represents  the  point  of  contact  of  two  other 
currents  which  in  their  higher  layers  advance  in  directions  perpendic- 
ular to  each  other.  That  is  the  origin  of  the  rotatory  movement,  and 
the  ulterior  progress  of  the  cyclone  will  necessarily  be  based  upon  the 
same  principles.  The  cyclone  being  thus  considered  as  the  result  of 
the  meeting  of  currents  at  different  points,  one  after  the  other,  may 
therefore  preserve  its  diameter  unchanged  for  a  considerable  period, 
and  it  may  even  diminish  in  size,  though  it  ordinarily  increases. 

"  It  is,  moreover,  quite  clear  that,  if  the  above  explanation  be  cor- 


332  THE  ATMOSPHERE. 

rect,  a  cyclone  which  turns  in  the  same  direction  may  originate  by  the 
interposition  of  some  mechanical  obstacle  in  the  route  of  the  current, 
as  it  travels  toward  the  high  latitudes  of  the  north — an  obstacle  which 
compels  this  current  to  assume  a  more  southerly  course  (that  of  a  south 
wind)  upon  its  eastern  than  upon  its  western  edge,  where  it  always  re- 
mains nearly  due  west.  This  was  what  happened,  to  cite  one  instance, 
during  a  cyclone  in  the  Bay  of  Bengal  on  the  3d,  4th,  -and  5th  of 
June,  1839." 

The  name  of  cyclone  is  therefore,  in  a  certain  measure,  the  geomet- 
rical designation  of  the  more  ancient  term  hurricane,  like  the  tornadoes 
which  are  seen  on  the  coasts  of  Africa,  like  the  typhoons  of  the  Chinese 
seas ;  the  great  tempests  that  occur  in  these  regions  are  of  the  same 
kind  as  the  cyclones  in  the  Atlantic.  Dampier,  the  prince  of  navigators, 
describes  the  approach  of  the  typhoon  with  that  accuracy  which  ren- 
ders all  his  works  so  reliable.  We  read  in  his  "Voyages"  (vol.  ii.,  p.  26): 

"  The  typhoons  are  a  special  kind  of  violent  tempests  which  blow 
along  the  coast  of  Tonquin  and  the  neighboring  shores  in  the  months 
of  July,  August,  and  September.  They  generally  occur  about  the 
period  of  full  moon,  and  are,  as  a  rule,  preceded  by  very  fine  weather, 
light  breezes,  and  a  clear  sky.  These  light  breezes  are  the  ordinary 
trade-winds,  which  blow  from  the  S.W.  at  this  season,  and  which  veer 
to  about  K  or  N.B.  Before  the  tempest  begins,  a  thick  cloud  forms  in 
the  N.E. ;  it  is  very  black  near  the  horizon,  copper-colored  toward  the 
summit,  and  gradually  lighter  in  color  toward  its  outside  edge,  which 
is  perfectly  white.  The  aspect  of  this  cloud  is  very  strange,  and  it  ap- 
pears sometimes  twelve  hours  before  the  storm  breaks.  When  it  be- 
gins to  move  very  rapidly,  the  wind  breaks  out  at  once,  augmenting  in 
force  with  great  suddenness,  and  blowing  with  great  violence  for  about 
twelve  hours.  It  is  often  accompanied  by  thunder  and  lightning,  and 
thick  rain.  When  the  wind  begins  to  diminish,  it  drops  very  sudden- 
ly for  about  an  hour,  after  which  it  recommences  to  blow  from  the 
S.W.  for  about  the  same  period  as  it  did  from  the  N.E.,  rain  falling 
as  before." 

The  course  followed  by  the  centre  divides  the  hurricane  into  two 
equal  parts,  but  into  parts  which  differ  from  each  other.  In  the  one 
the  movement  of  rotation  and  that  of  translation  have  the  same  direc- 
tion; in  the  other,  on  the  contrary,  the  direction  of  translation  and 
that  of  rotation  are  different.  It  follows  that  at  an  equal  distance  from 
the  centre  there  is  much  more  wind  in  the  first  hemicycle  than  in  the 


THE  POWER  OF  THE  AIR.  333 

second ;  hence  the  name  of  dangerous  hemicycle  is  given  to  the  one,  and 
that  of  manageable  hemicycle  to  the  other. 

In  the  northern  hemisphere  the  cyclone  turns  from  right  to  left— 
that  is  to  say,  that  an  observer  placed  in  the  centre  of  the  whirlwind 
would  see  the  wind  pass  before  him  from  right  to  left.  The  dangerous 
hemicycle  will  be  to  his  right  if  he  follows  the  same  route  as  the  centre 
of  the  hurricane,  and  the  manageable  hemicycle  to  his  left. 

In  the  southern  hemicycle,  on  the  contrary,  the  hurricane  turns  from 
left  to  right ;  the  dangerous  hemicycle  is  to  the  left,  and  the  manage- 
able hemicycle  to  the  right  of  the  line  through  which  the  centre  passes 
if  he  follows  the  same  direction  as  the  hurricane. 

The  direction  of  the  wind  observed  at  a  given  point  of  the  cyclone  is 
very  near  to  a  tangent  drawn  to  the  concentric  circle  upon  the  circum- 
ference of  which  it  is  placed.  Consequently,  it  is  always  nearly  per- 
pendicular to  the  radius  drawn  from  this  point  to  the  centre  of  the  con- 
centric circle  or  cyclone.  Now,  the  law  of  gyration  indicates  that  if 
one  faces  the  wind  the  centre  will  be  to  the  right  in  the  northern  hem- 
isphere, and  to  the  left  in  the  southern,  but  always  at  right  angles  to 
the  direction  of  the  wind. 

It  is  upon  this  latter  fact,  which  numerous  observations  place  beyond 
a  doubt,  that  all  the  theories  as  to  the  means  of  avoiding  the  centre  of  a 
cyclone,  by  moving  away  from  the  line  which  it  takes,  are  based.  The 
nearer  the  centre,  the  more  violent  the  wind,  and  the  more  sudden  its 
variations.  The  sea  is  also  roughest  at  the  centre,  being  subject,  at  very 
short  intervals,  to  violent  gusts  of  wind  from  all  directions  —  and  this 
after  having  been  under  the  influence  of  comparatively  regular  winds 
which  have  had  time  to  cause  a  heavy  swell,  and  to  impart  to  the  water 
a  direction  different  from  that  of  the  wind.  Hence  arise  the  short  and 
chopping  waves  which  assail  a  vessel  on  all  sides  at  once. 

It  is  easy,  however,  to  avoid  the  place  over  which  the  centre  of  a  cy- 
clone passes. 

Let  us  suppose  that  the  centre  of  a  cyclone  is  coming  toward  a  vessel. 
It  will  pass  over  this  vessel,  or  to  the  right  or  to  its  left.  If  it  is  about 
to  pass  over,  its  direction  in  respect  to  the  ship  will  not  change;  but 
then  the  direction  of  the  wind,  which  is  always  perpendicular,  will  not 
change  either,  and  the  crew  will  find  the  wind  increase  in  violence  with- 
out changing  direction. 

If  the  centre  pass  to  the  right  of  the  vessel,  it  will  shift  slightly  to- 
ward the  right.  Its  direction  will  vary  from  left  to  right ;  but  that  of 


334  THE  ATMOSPHERE. 

the  wind,  which  is  connected  with  the  first,  will  vary  in  the  same  direc- 
tion— that  is,  from  left  to  right. 

The  exact  opposite  will  take  place  if  the  centre  pass  to  the  left  of  the 


Thus,  if  the  wind  increases  without  changing  its  direction,  the  vessel 
will  be  upon  the  line  along  which  the  centre  passes ;  if  the  wind  veers 
from  left  to  right,  it  will  be  to  the  left  of  this  line ;  if  the  wind  changes 
from  right  to  left,  it  will  be  to  the  right  of  the  same  line. 

From  these  laws  regulating  cyclones,  we  may  gather  that  the  worst 
position  in  which  a  vessel  can  be  is  that  which  leads  to  the  centre  of  the 
hurricane,  and,  to  avoid  this,  all  efforts  should  be  directed. 

The  premonitory  signs  of  the  cyclone  are :  Some  days  before  the  hur- 
ricane, both  at  sunrise  and  sunset,  the  clouds  assume  a  reddish  and  or- 
ange hue,  which  becomes  reflected  upon  the  sea ;  and  it  is  this  which 
renders  them  so  brilliant  and  splendid,  and  which  inspires  with  such 
sentiments  of  admiration  those  who  do  not  dream  of  the  imminent  dan- 
ger which  they  foreshadow. 

As  the  cyclone  approaches,  this  reddish  tint  gets  deeper  in  color; 
then  a  black  and  deep  band  extends  across  the  sky ;  the  edges  of  the 
cumulus  are  of  a  copper  hue,  imparting  to  the  sea  and  the  land  an  analo- 
gous glitter  which  makes  the  atmosphere  look  as  if  it  were  on  fire ;  the 
sea-birds  fly  rapidly  inland  to  seek  shelter  from  the  fury  of  the  tempest 
whicl}  they  have  an  instinct  is  coming,  thus  hoping  to  escape  the  death 
which  would  overtake  them  at  sea. 

But  of  all  the  premonitory  signs  of  the  tempest,  the  surest  and  the 
easiest  to  interpret  is  the  movement  of  the  mercury  of  the  barometer. 

As  the  pressure  of  the  air  gradually  diminishes  frqm  the  circumfer- 
ence to  the  centre  of  the  whirlwind,  the  approach  of  the  phenomenon  is 
always  made  evident  by  a  fall  of  the  barometer.  This  same  symptom 
characterizes  the  tempests  in  our  temperate  regions,  which  are  in  reali- 
ty, so  to  speak,  the  continuations  of  the  oceanic  cyclone. 

The  barometer  begins  to  fall  twelve,  twenty  -  four,  and  even  forty- 
eight  hours  before  the  arrival  of  the  cyclone. 

An  oppressive  calm,  accompanied  by  a  suffocating  air,  prevails  for 
four-and-twenty  hours ;  Nature  seems  to  be  collecting  all  her  strength 
to  accomplish  the  work  of  devastation. 

Whatever  may  be  the  course  taken  by  the  hurricane,  the  point  near- 
est to  its  centre  is  known  when  the  barometer  reading  ceases  to  de- 
crease. Then,  for  a  space  of  two  or  three  hours,  the  reading  of  the  ba- 


THE  POWER  OF  THE  AIR. 


335 


rometer  will  rise  and  fall  every  half-hour,  without  making  any  decided 
movement  up  or  down,  this  being  a  certain  sign  of  proximity  to  the 
centre,  that  the  heaviest  blasts  have  been  felt,  and  that  the  violence  of 
the  storm  will  gradually  abate. 

The  total  decrease  of  the  barometer  is  proportionately  greater  as  the 
central  rarefaction  is  more  complete,  and  this  rarefaction,  chiefly  caused 
by  centrifugal  force,  augments  in  ratio  to  the  increase  of  the  rotatory 
movement,  which  causes  hurricanes  to  be  so  violent.  The  barometer, 
therefore,  declines  in  proportion  as  the  violence  of  the  wind  becomes 
more  intense,  and  the  most  disastrous  hurricanes  are  those  which  influ- 
ence it  to  the  greatest  degree. 

The  rarefaction  of  the  atmosphere  at  the  centre  of  cyclones  is  clearly 
proved  by  the  following  table,  taken  from  the  register  of  a  barometer 
during  the  hurricane  that  passed  over  St.  Thomas  on  the  2d  of  August, 
1837,  when  the  central  calm  occurred  at  8  P.M.  : 


Align 

st  2,    6       A 
2       P 
3-20 
4-45 
5-45 
6-30 
6-35 
7 
7-10 
7-22 
7-35 

•  M     .  . 

Inches. 
.      29  '922 

g  Hurricane  to  the  N.W. 

August  2, 

August  3, 
0-89  inch! 

7-50 
8'20 
8-22 
8-38 
8-50 
9 
9-25 
9-50 
11 
2 
9 

Inches. 
28-032) 

M  

......   29-764 

28  '032  j 

29-646 

4, 

28-3861 

«     t 

29-4891 

n 

28*583 

i 

29-292 

,4 

28  '780 

, 

29*134 

lt 

28  "938 

( 

28-898 

44 

29-213 

i 

28-780 

„ 

29*410 

<  ;ZI 

28-583 
28-268 
28-111, 
Variat 

„ 

29'607 

A.M  

29*725 

29-922 

These  large  perturbations  of  the  air  are  perhaps,  next  to  great  vol- 
canic eruptions,  the  most  fearful  phenomena  that  take  place  upon  the 
globe,  and,  as  Eeclus  remarks  in  his  work  upon  the  "Earth,"  we  can  not 
be  astonished  that,  in  Hindoo  mythology,  Rudra,  the  chief  of  winds  and 
storms,  should  have  become,  under  the  name  of  Siva,  the  god  of  de- 
struction and  death.  For  some  days  before  the  outbreak  of  a  hurri- 
cane, Nature,  desolate  and  gloomy,  seems  to  foresee  a  disaster.  The 
small  white  clouds  which  travel  in  the  air  with  the  anti-trade-winds  are 
concealed  by  a  yellowish  vapor ;  the  stars  are  surrounded  by  halos  with 
a  vague  iris,  and  by  heavy  banks  of  clouds  which,  about  evening,  are 
beautifully  tinted  with  purple  and  gold.  The  air  is  suffocating,  as  if  it 
issued  from  the  mouth  of  a  furnace.  The  cyclone,  which  is  already  re- 
volving in  the  upper  regions,  gradually  descends.  Jagged  remnants  of 


336  THE  ATMOSPHERE. 

reddish  or  black  clouds  are  borne  furiously  along  by  the  tempest,  which 
plunges  rapidly  through  space,  and  the  column  of  mercury  descends  in 
the  barometer.  Soon  an  obscure  mass  becomes  visible  in  the  stormy 
part  of  the  sky,  and,  increasing  in  size,  gradually  covers  the  firmament 
with  a  veil  of  darkness  and  a  blood-red  glitter.  This  is  the  cyclone, 
which  is  swooping  down  upon  the  earth,  and  a  terrible  silence  succeeds 
the  moaning  of  the  sea  and  of  the  skies. 

In  the  early  part  of  the  cyclone  a  strange,  dull  sound  is  sometimes 
heard,  "  with  a  noise  like  that  of  the  wind  in  very  old  houses  during 
winter  nights." — Piddington.  The  gusts  which  rend  the  air  during  the 
time  the  cyclone  continues  are  said  to  create  a  noise  like  that  of  the 
roaring  of  wild  beasts,  a  tumult  of  countless  voices,  and  cries  of  terror. 
At  the  points  where  the  centre  passes,  a  formidable  sound  like  the  dis- 
charge of  artillery,  an  incessant  rolling  of  thunder  (the  voice  of  the  hur- 
ricane, as  it  in  fact  is),  is  heard  above  all  others. 

The  progress  of  the  winds  meets  with  a  certain  degree  of  resistance 
upon  land,  but  the  destruction  caused  is  none  the  less  terrible.  Build- 
ings which  lie  in  their  path  are  overturned ;  the  waters  of  a  stream  are 
driven  back  toward  their  source;  isolated  trees  are  torn  up  by  their 
roots ;  forests  are  bent  down  as  if  they  formed  but  one  compact  mass, 
and  their  branches  and  leaves  are  scattered ;  the  grass  is  swept  off  the 
ground.  In  the  track  of  the  hurricane  fly  countless  debris  like  the  flot- 
sam carried  along  by  a  stream.  Generally  speaking,  the  action  of  elec- 
tricity is  superadded  to  the  violence  of  the  air  in  motion,  and  helps  to 
augment  the  ravages  of  the  tempest :  sometimes  flashes  of  lightning  are 
so  rapid  that  they  descend  like  a  sheet  of  flame ;  the  clouds,  and  even 
drops  of  rain,  emit  light ;  the  electric  tension  is  so  great  that,  according 
to  Reid,  sparks  have  been  seen  to  fly  from  the  body  of  a  negro.  A 
whole  forest  in  the  Island  of  St.  Vincent  was  killed  without  the  trunk 
of  a. single  tree  being  blown  down.  In  Europe,  too,  upon  the  shores  of 
Lake  Constance,  many  trees  were  skinned  of  their  bark,  though  they 
still  remained  upright  in  the  ground. 

The  most  terrible  cyclone  of  modern  times  is  probably  that  which  oc- 
curred on  October  10,  1780,  which  has  been  specially  called  the  Great 
Hurricane,  and  which  seems  to  have  embodied  all  the  horrible  scenes 
that  attend  a  phenomenon  of  this  kind.  Starting  from  Barbados,  where 
trees  and  houses  were  all  blown  down,  it  ingulfed  an  English  fleet 
anchored  before  St.  Lucie,  and  then  ravaged  the  whole  of  that  island, 
where  six  thousand  persons  were  buried  beneath  the  ruins.  From 


THE  POWER  OF  THE  AIR.  337 

thence  it  traveled  to  Martinique,  overtook  a  French  transport  fleet,  and 
sunk  forty  ships  conveying  four  thousand  soldiers.  "  The  vessels  disap- 
peared;" such  is  the  laconic  language  in  which  the  governor  reported 
this  disaster.  Farther  north,  St.  Domingo,  St. Vincent,  St.  Eustache,  and 
Porto  Eico  were  also  devastated,  and  most  of  the  vessels  that  were  sail- 
ing in  the  track  of  the  cyclone  were  lost,  with  all  on  board.  Beyond 
Porto  Rico  the  tempest  turned  north-east  toward  Bermuda,  and  though 
its  violence  gradually  decreased,  it  nevertheless  sunk  several  English 
vessels.  This  hurricane  was  quite  as  destructive  inland.  Nine  thou- 
sand persons  perished  in  Martinique,  and  a  thousand  at  St.  Pierre,  where 
not  a  single  house  was  left  upstanding,  for  the  sea  rose  to  a  height  of 
twenty-five  feet,  and  150  houses  that  were  built  along  the  shores  were 
ingulfed.  At  Port  Royal,  the  cathedral,  seven  churches,  and  1400 
houses  were  blown  down ;  1600  sick  and  wounded  were  buried  beneath 
the  ruins  of  the  hospital.  At  St.  Eustache,  seven  vessels  were  dashed 
to  pieces  against  the  rocks;  and  of  the  nineteen  which  lifted  their  anchors 
and  sailed  to  sea,  only  one  returned.  At  St.  Lucie  the  strongest  build- 
ings were  torn  up  from  their  foundations;  a  cannon  was  hurled  to  a 
distance  of  more  than  thirty  yards,  and  men  as  well  as  animals  were 
lifted  off  their  feet  and  carried  several  yards.  The  sea  rose  so  high  that 
it  destroyed  the  fort,  and  drove  a  vessel  against  the  hospital  with  such 
force  as  to  stave  in  the  walls  of  that  building.  Of  the  600  houses  at 
Kingstown,  in  the  Island  of  St. Vincent,  fourteen  alone  remained  intact, 
and  the  French  frigate  Junon  was  lost. 

In  the  Leeward  Islands,  the  inhabitants  of  the  Government  Palace 
took  refuge  in  the  centre  of  the  building  during  the  height  of  the  storm, 
thinking  that  the  immense  thickness  of  the  walls  (nearly  a  yard)  and 
their  circular  shape  would  preserve  them  from  the  fury  of  the  wind. 
At  half-past  eleven  they  were  obliged  to  repair  to  the  cellar,  as  the  wind 
had  penetrated  everywhere-  and  lifted  off  the  roof.  The  water  rising 
there  to  the  height  of  more  than  a  yard,  they  were  driven  into  the  bat- 
tery and  protected  themselves  behind  cannons,  some  of  which  were 
driven  from  their  places  by  the  force  of  the  wind.  The  hurricane  was 
so  violent  that,  seconded  by  the  sea,  it  carried  a  twelve-pounder  a  dis- 
tance of  more  than  400  feet.  (This  cannon  was,  it  must  be  supposed, 
upon  its  carriage,  which  had  wheels.)  By  the  light  of  day  the  country 
looked  as  it  does  in  midwinter;  there  was  not  a  single  leaf,  or  even  a 
branch,  remaining  upon  the  trees.  Human  passions  are  quelled  in  pres- 
ence of  such  a  war  of  the  elements.  When  the  Laurier  and  the  Andro- 

22 


338  THE  ATMOSPHERE. 

mMe  were  lost  at  Martinique,  the  Marquis  de  Bouilld  set  at  liberty  the 
five-arid-twenty  English  sailors  who  had  survived  the  shipwreck,  writ- 
ing to  the  Governor  of  St.  Lucie  that  he  was  unwilling  to  retain  prison- 
ers men  who  had  fallen  into  his  hands  during  a  disaster  to  which  every 
one  was  liable. 

The  last  memorable  tempest  is  that  of  March  3,  1869,  when  the  three- 
masted  vessel  La  Lerida,  of  Nantes,  was  lost  off  Le  Havre,  on  her  way 
from  Haiti.  On  March  2,  at  10  A.M.,  this  vessel,  which  for  two  hours 
had  been  struggling  against  a  fearful  sea,  approached  the  jetty,  where  a 
tremendous  current,  the  force  of  which  was  further  increased  by  the 
north-west  wind,  raised  up  an  insuperable  barrier.  The  vessel  soon  felt 
the  first  shock  of  the  current  which,  two  hours  later,  would  have  had 
little  effect.  Hitherto  it  had  managed  to  sail  with  the  wind  blowing 
aft,  and  this  manoeuvre,  diminishing  its  speed,  left  it  almost  at  the  mercy 
of  the  hostile  elements.  A  feeling  of  despair  came  over  the  spectators, 
most  of  whom  were  seamen.  They  saw  that  this  movement  had  grave- 
ly compromised  the  chances  of  the  vessel's  escape.  The  captain  tried 
another  effort.  He  endeavored  to  luff,  so  as  to  run  his  ship  into  the 
mouth  of  the  Seine,  but  this  was  attempted  too  late.  One  last  chance 
remained — the  two  anchors  were  cast  out,  but  they  did  not  grip !  Still, 
for  a  moment  there  seemed  room  for  hope ;  the  anchors  had  caught,  but 
the  heavy  sea  snapped  the  chains.  It  was  all  over  in  less  time  than  it 
takes  to  write  these  lines :  the  Lerida,  at  the  mercy  of  the  waves,  ran 
against  the  angle  of  the  bastion,  which  stove  in  its  poop  and  bulwarks. 
The  only  thing  that  remained  was  to  endeavor  to  save  the  crew.  For- 
tunately the  ship  was  near  enough  to  land  to  admit  of  ropes  being 
thrown  to  them,  and  all  were  rescued  save  two,  who,  losing  their  pres- 
ence of  mind,  clung  to  a  rope  that  was  not  strong  enough  to  bear  their 
weight.  The  captain,  who  had  staid  on  the  vessel  last  of  all,  had  scarce- 
ly left  when  she  went  down. 

I  may  finally  add  that  in  the  torrid  zone,  and  in  climates  where  tem- 
perature is  high,  hurricanes  are  numerous,  and  extremely  violent;  in 
our  temperate  climates  they  are  at  once  rarer  and  less  violent;  and  in 
the  polar  regions,  the  great  atmospheric  disturbances,  which  occur  very 
frequently,  are  limited  to  winds  which,  though  tempestuous,  do  not  con- 
stitute a  hurricane. 


TROMBES,  WHIRLWINDS,  OR  WATER-SPOUTS.  339 


CHAPTER  VI. 

TROMBES,  WHIRLWINDS,  OR  WATER-SPOUTS. 

AMONG  the  chief  phenomena  which  disturb  the  apparently  regular 
order  and  harmony  of  Nature,  scattering  terror  and  desolation  in  their 
paths,  there  is  one  remarkable  for  its  peculiar  and  colossal  form,  for  the 
forces  which  it  seemingly  obeys,  for  the  unknown  and  apparently  con- 
tradictory laws  which  it  follows,  and  for  the  disasters  which  it  causes. 
These  disasters  are  themselves  accompanied  by  such  strange  circum- 
stances, that  their  origin  can  not  be  confounded  with  the  other  phenom- 
ena which  prove  so  fatal  to  man.  This  meteor,  fortunately  rare  in  this 
part  of  the  world,  is  designated  in  France  by  the  general  term  trombe. 

Previous  to  Peltier's  explanation  of  this  peculiar  atmospheric  phe- 
nomenon, it  was  imperfectly  known.  We  are  now  able  to  describe 
with  precision  its  nature  and  its  character ;  and  we  know  that  a  trombe 
is  a  column  of  air  which  generally  turns  rapidly  upon  its  own  axis, 
and  which  revolves  comparatively  slowly,  for,  as  a  rule,  a  person  can 
keep  up  with  it  at  a  walking  pace.  This  whirling  column  of  air  is 
both  caused  and  set  in  motion  by  electricity.  The  sometimes  violent 
wind  which  its  movement  produces,  and  which  acts  so  disastrously,  as 
we  shall  presently  see,  is  not  the  result  of  atmospheric  currents  upon  a 
large  scale,  as  with  the  cyclones,  but  is  confined  to  very  limited  dimen- 
sions. The  trombes  are  often  only  a  few  yards  in  diameter,  but  their 
force  is  very  great.  They  sweep  the  soil  over  which  they  pass,  de- 
stroying trees  and  houses  so  completely  that  sometimes  nothing  re- 
mains upright  in  the  track  along  which  they  have  passed.  This  phe- 
nomenon generally  has  its  origin  as  follows : 

By  virtue  of  considerable  electric  tension,  the  lower  surface  of  a 
stormy  cloud  descends  toward  the  earth  in  the  shape  of  a  cylinder,  or 
rather  of  a  cone,  like  a  great  speaking-tube,  the  top  of  which  is  lost  in 
the  clouds,  while  the  orifice  is  relatively  close  to  the  surface.  This  re- 
versed cone  may  be  more  or  less  developed,  more  or  less  different  in 
shape,  according  to  the  special  condition  of  the  clouds  or  the  locality. 
.That  which  is  always  present  is  a  connecting  link  of  vapor  between  the 
clouds  and  the  earth. 


340  THE  ATMOSPHERE. 

Beneath  the  cloudy  column  there  is  a  great  agitation  upon  the  sea  or 
upon  the  ground.  Sailors  compare  it  to  a  boiling  process  which  would 
emit  vapor  and  streams  of  liquid  sheaves.  Upon  land  the  dust  of  the 
roads  and  light  substances  form  an  analogous  kind  of  smoke.  In  a 
short  space  of  time  the  lower  whirlwind  rises  sufficiently  high,  and  the 
upper  column  descends  low  enough  to  admit  of  their  joining  and  being 
fused  into  one  and  the  same  column,  which  is  thicker  at  its  higher  than 
at  its  lower  part,  and  which  is  often  transparent  like  a  tube,  within 
which  vapor  can  be  seen  rising  and  falling. 

When  the  centre  of  waters  raised  over  the  sea  is  more  compact,  it 
appears  like  a  pillar  placed  to  sustain  the  descending  column.  There 
proceeds  from  this  column  a  noise  which  varies  considerably,  from 
what  seems  like  the  hissing  of  a  serpent  to  the  noise  of  heavy  wagons 
being  driven  over  stony  roads.  This  noise  is  much  more  pronounced 
on  land  than  at  sea. 

The  germs  of  destruction  seem  to  be  embodied  in  this  singular  for- 
mation. The  trombe  advances  slowly,  to  all  appearances,  blowing  vio- 
lently, writhing  convulsively,  leaving  its  mark  upon  all  the  productions 
of  nature  and  humanity,  and  rending  into  atoms  all  that  oppose  its  ad- 
vance. The  disasters  caused  by  this  formidable  agent  show  that  its 
pressure  is  sometimes  as  much  as  eighty  to  one  hundred  pounds  to  the 
square  foot.  Flocks  of  cattle,  men,  and  even  rivers,  are  lifted  to  an  im- 
mense height.  The  roofs  of  houses  are  carried  into  the  air;  walls  are 
leveled  by  the  sudden  violence  of  an  irresistible  pressure.  To  judge 
of  the  force  of  this  strange  phenomenon,  let  us  consider  some  of  its 
most  remarkable  effects. 

Take,  for  instance,  two  trombes  which  were  observed  to  the  south  of 
Paris,  May  16th,  1806,  from  one  to  two  P.M.,  and  which  are  particu- 
larly good  instances  of  these  phenomena.  Peltier  copies  them  from 
Professor  Debrun.  They  may  be  termed  the  Paris  trombes.  "The 
first  began  about  one  o'clock,  and  seemed  to  be  at  least  twelve  feet 
wide  at  its  base  near  the  cloud,  like  that  of  a  cone  turned  upside  down. 
It  then  became  successively  fifteen,  twenty,  and  forty  feet  long.  The 
lower  it  descended  the  more  pointed  became  its  conical  form,  for, 
when  it  first  left  the  cloud,  it  formed  a  perfect  cone.  Gradually  in- 
creasing in  length  and  decreasing  in  breadth,  it  finally  became  no  big- 
ger than  a  man's  arm. 

"This  whirlwind  traveled  very  slowly  toward  the  south,  then  west 
and  south-west,  and  seemed  to  be  suspended  over  the  last  houses  of 


THOMBES,  OB  WHIRLWINDS.  34^ 

the  Faubourg  St.  Jacques,  then  above  the  plain  of  Montrouge  and 
Montsouris.  It  was  of  a  gray  and  white  color  like  ordinary  clouds, 
and  stood  out  very  clearly  against  the  background  of  the  darker 
clouds. 

"What  struck  me  most  was  that  it  formed  a  long  tube,  partially  semi- 
transparent,  gradually  making  several  curves  and  inflections,  something 
like  a  long  flexible  piece  of  gut,  in  which  I  saw  vapors  mounting  with 
an  undulating  movement,  like  smoke  which  might  ascend  a  stove-pipe 
in  glass.  The  most  curious  fact  was,  that  the  ascent  of  the  vapors  was 
much  more  marked  and  active  toward  the  lower  part,  which  was  then 
about  300  or  400  feet  above  the  ground. 

"  As  the  cloud  which  formed  the  head  of  the  column  advanced,  the 
main  mass  described  a  curve  and  followed  it,  becoming  elongated  by 
1500  or  1600  fathoms,  and  remaining  attached  to  it.  But  when  the 
column  became  extremely  long,  and  consequently  very  slight  in  vol- 
ume, and  when  it  formed  an  angle  of  20°  or  25°  with  the  horizon,  then 
the  main  body  of  the  column  began  to  curl  off  (or  become  detached). 
This  whirlwind,  when  its  inflection  was  most  pronounced,  seemed  to 
have  its  head  over  Chatillon  and  its  tail  over  Arcueil ;  but  while  the 
head  of  the  column  was  moving  forward,  I  remarked  that  the  lower 
part  seemed  to  be  attracted  by  the  valley  of  Arcueil,  and  that  it  had 
great  difficulty  in  emerging  from  it. 

"It  lasted  for  more  than  three-quarters  of  an  hour,  and  went  off  to  a 
point  at  last.  Its  upper  part  seemed  to  me  to  work  its  way  back  into 
the  cloud  whence  it  started,  though,  as  it  was  then  at  a  great  distance 
to  the  S.S.W.  of  Paris,  and  very  small  in  volume,  I  could  not  affirm 
this  positively. 

"About  twenty  minutes  after  the  formation  of  this  whirlwind  I  saw 
a  second,  which  did  not,  indeed,  present  so  many  marked  peculiarities 
as  the  first,  but  which  was  far  more  majestic  in  appearance.  It  was 
produced  by  a  cloud,  not  nearly  so  high  in  the  air  as  that  which  caused 
the  first,  and  it  was  visible  above  the  Rue  du  Faubourg  St.  Jacques 
and  the  Observatory.  It  was  of  a  grayish  hue,  and  was  traversed  from 
top  to  bottom  by  a  tube  as  luminous  as  the  moon.  I  saw  the  vapors 
rising  and  falling  in  the  lower  part  of  it  very  distinctly.  At  short- in- 
tervals the  body  of  this  whirlwind  lengthened  and  shortened,  and  some- 
times rapidly.  It  passed  before  the  first,  and  seemed  not  to  be  more 
than  from  1600  to  2000  paces  to  the  north ;  but  the  first,  just  before 
it  disappeared,  traveled  much  more  rapidly  southward.  It  followed 


342  THE  ATMOSPHERE. 

about  the  same  direction  as  the  first,  and  its  lower  part  curved  slightly 
toward  the  west. 

"There  was  a  thunder-clap  from  a  cloud  not  very  far  from  the  whirl- 
winds, especially  from  the  second ;  they  did  not  seem  to  be  in  any  way 
affected  by  it.  We  judged,  from  the  loudness  of  the  report,  that  the 
lightning  had  struck  the  ground.  Drops  of  rain  as  large  as  a  man's 
thumb  fell  at  the  point  where  I  was  standing,  followed  by  hailstones  as 
big  as  nuts. 

"The  second  whirlwind  gradually  made  its  way  back  to  the  cloud 
out  of  which  it  had  proceeded,  and  by  which  it  was  rapidly  re-absorbed. 
It  had  not  lasted  altogether  more  than  five-and-twenty  minutes." 

These  whirlwinds  were,  as  will  have  been  gathered,  harmless.  They 
do  not  seem  to  have  touched  the  ground;  but  there  is  no  doubt  they 
would  have  proved  more  dangerous  to  any  balloon  which  might  have 
approached  them. 

We  now  come  to  trombes  of  another  kind,  the  passage  of  which  along 
the  surface  of  the  ground  leaves  unmistakable  traces  of  their  power. 

"At  1-30  P.M.  of  the  6th  of  July,  1822,  in  the  plain  of  Assonval,  six 
leagues  distant  from  St.  Omer  and  Boulogne,  the  clouds,  coming  from 
different  points  of  the  horizon,  suddenly  effected  a  junction,  and  cover- 
ed with  one  mass  the  whole  sky.  Directly  afterward,  a  thick  vapor, 
with  the  bluish  hue  of  burning  sulphur,  was  seen  to  descend  from  this 
cloud.  It  formed  a- reversed  cone,  the  base  of  which  touched  the  cloud. 
The  lower  part  of  the  cone,  which  reached  to  the  ground,  soon  formed 
an  oblong  mass  of  about  thirty  feet  detached  from  the  cloud,  revolving 
very  rapidly. 

"As  it  rose,  it  emitted  a  sound  like  that  caused  by  the  bursting  of  a 
large  shell,  leaving  an  indentation  upon  the  ground  about  twenty-five 
to  thirty  feet  in  circumference,  and  to  a  depth  of  three  or  four  feet  in 
the  middle.  When  at  about  a  hundred  yards  from  its  point  of  depart- 
ure, and  moving  in  an  easterly  direction,  the  whirlwind  blew  down  a 
barn,  and  shook  a  solidly-constructed  house  with  the  force  of  an  earth- 
quake. On  its  way,  it  rooted  up  a  group  of  very  large  trees,  which 
were  found  lying  in  many  different  directions,  showing  that  the  whirl- 
wind was  revolving  while  advancing.  Others  had  their  topmost  branch- 
es torn  off,  and  some  of  these  were  found  hanging  to  the  tops  of  other 
trees  sixty  or  seventy  feet  from  the  ground. 

"The  whirlwind  then  went  a  distance  of  two  leagues  without  touch- 
ing the  soil,  tearing  off  large  branches  of  trees  which  it  scattered  right 


TROMBES,  OR  WHIRLWINDS.  343 

and  left :  reaching  the  corner  of  a  wood,  it  carried  off  the  tops  of  some 
large  o'aks  which  were  blown  over  the  village  of  Vendome,  situated  at 
the  foot  of  the  hill  to  the  east  of  the  forest. 

"  Globes  of  sulphurous  vapor  were  from  time  to  time  emitted  from 
the  centre  of  this  whirlwind,  and  the  noise  which  it  made  was  like  that 
of  a  heavy  carriage  driven  rapidly  over  paving-stones.  Each  time  that 
a  globe  of  fire  or  vapor  was  emitted  there  was  an  explosion  like  that 
of  a  gun,  the  wind,  which  was  very  violent,  adding  a  wild  shriek.  Af- 
ter having  torn  up  the  soil  and  every  thing  which  resisted  it,  the  whirl- 
wind rose  into  the  air  and  went  on  to  a  distance  of  a  league  and  a  half, 
where  it  recommenced  its  ravages. 

"  Thence  it  reached  the  valleys  of  Witernestre  and  Lambre.  In  the 
first  of  these  villages,  composed  of  forty  houses,  only  eight  were  left  in- 
tact ;  and  it  was  noticed  that  the  gables  and  walls  of  the  houses  were 
blown  in  all  directions — showing  that  the  wind  had  blown  from  every 
quarter. 

"The  disasters  which  it  caused  at  Lambre  were  not  less  extensive. 
Several  persons  remarked  the  circular  progress  of  the  trombe,  its  sul- 
phurous hue,  and  the  focus  of  flaming  fire  which  issued  with  the  sparks 
of  bituminous  vapor.  The  trees  around  the  church  were  broken  and 
uprooted,  the  cure's  house  carried  away,  and  eighteen  others,  mostly  of 
brick,  were  snapped  off  at  their  foundations,  with  the  curious  phenome- 
non of  the  walls  falling  outward." 

The  following  whirlwind  was  not  less  remarkable:  At  3  P.M.  on  the 
26th  of  August,  1823,  after  some  calm  and  warm  weather,  a  whirlwind 
appeared  at  Eouvier  (Eure  et  Loir).  It  was  preceded  by  a  black  cloud 
from  the  S.  W.,  followed  by  others  of  a  yellowish  hue,  with  intermittent 
thunder  and  hail.  Apparently  touching  the  cloud  at  its  summit,  and 
with  its  base  on  a  level  with  the  ground,  it  threw  down  every  thing  in 
its  passage,  hurling  the  soil  and  trees  to  a  great  distance.  The  whirl- 
wind was  of  a  dark  yellow  color — due,  no  doubt,  to  the  dust  and  other 
substances  which  it  carried  off.  The  leaves  of  the  hedges  and  trees 
which  were  not  blown  off  were  dried  as  if  by  fire.  In  the  hamlet  of 
Marchefroid,  where  it  continued  only  a  minute,  it  destroyed  fifty-three 
houses.  The  inhabitants  heard  no  thunder,  nor  did  much  hail  fall.  A 
child  of  three  years  of  age  was  killed.  A  deep  wound  was  found  in  its 
neck,  but  it  was  impossible  to  tell  what  body  had  caused  it.  In  the 
valley  of  St.  Ouen,  the  meteor  destroyed  a  range  of  trees  extending  800 
feet,  and  then  moved  toward  Mantes,  extending  over  a  width  of  from 


344  THE  ATMOSPHERE. 

forty  to  fifty  fathoms.  Whole  houses  were  swept  away,  and  in  the  di- 
rection followed  by  the  whirlwind  branches  of  trees  were  found  scat- 
tered on  all  sides.  Trees  were  snapped  off  at  a  height  of  four,  six,  and 
ten  feet  from  the  ground  in  the  valley — a  fact  which  would  lead  one  to 
suppose  that  the  tempest  there  did  not  reach  quite  so  low  as  the  ground. 
In  one  instance  the  destruction  was  very  regular.  The  four  walls  of  a 
garden,  built  of  solid  stone,  were  blown  down,  each  wall  in  a  straight 
line,  and  as  if  the  stones  had  been  placed  there  for  constructing  a  wall. 
The  body  of  a  three-horse  wagon  loaded  with  grain  was  blown  off  the 
carriage  and  carried  on  to  the  top  of  a  building,  the  roof  of  which  it 
stove  in.  Pieces  of  the  wood-work  of  the  wagon  were  found  upon  the 
other  side  of  the  building.  The  grain  had  disappeared,  and  the  horses, 
though  uninjured,  had  been  entirely  stripped  of  their  harness. 

The  following  case  is  equally  remarkable :  On  the  26th  of  August, 
1826,  the  neighborhood  of  Carcassonne  was  visited  by  an  enormous 
column  of  fire  which,  sweeping  along  the  surface  of  the  soil,  destroyed 
every  thing  that  lay  in  its  passage.  A  young  man  was  carried  off  by 
it  into  the  air  and  hurled  head  foremost  against  a  rock*.  Fourteen 
sheep  were  taken  off  their  legs  and  asphyxiated.  This  column  of  air 
and  fire  overturned  walls,  displaced  enormous  rocks,  uprooted  the  largest 
trees,  and  did  great  damage  to  a  very  solidly-constructed  country  house. 
The  air,  wherever  it  passed,  was  impregnated  with  sulphur. 

Among  the  whirlwinds  which  have  left  traces  of  great  destruction 
behind  them  must  be  cited  that  of  Monville  on  the  19th  of  August, 
1845.  The  valley  in  question,  which  is  so  attractive  a  point  in  the 
railway  journey  between  Kouen  and  Dieppe,  was  visited  at  about  1 
P.M.,  the  weather  being  hot  and  oppressive,  by  a  whirlwind  of  a  very 
remarkable  kind.  The  large  mills  existing  at  Monville  were  suddenly 
enveloped  and  blown  down.  The  factory,  in  which  hundreds  of  wom- 
en were  at  work,  fell  in,  amidst  a  sudden  discharge  of  electricity,  and 
they  were  buried  beneath  its  ruins.  Some  of  them  who  escaped  death 
were  unable  to  understand  what  had  happened,  and  believed  that  the 
end  of  the  world  had  arrived.  Men  were  hurled  over  hedges;  others 
were  cut  to  pieces  by  the  machinery  which  was  whirled  about  in  the 
air;  others,  without  being  actually  hurt,  were  so  terrified  that  they  died 
from  the  effects  of  the  fright  in  the  course  of  a  few  days.  Whole  rooms 
and  walls  were  turned  upside  down,  so  as  to  be  no  longer  recognizable. 
At  other  points  the  buildings  were  literally  pulverized,  and  their  site 
swept  clean.  Planks  measuring  a  yard  long,  five  inches  wide,  and  near- 


TROMBES,  OR  WHIRLWINDS.  347 

ly  half  an  inch  thick,  archives  and  papers,  were  carried  to  distances  of 
fifteen  to  twenty-five  miles,  almost  to  Dieppe.  Trees  situated  in  the 
track  of  the  storm  were  blown  down  and  dried  up.  The  extent  of  the 
ground  thus  devastated  was  as  much  as  nine  miles,  increasing  from  100 
yards  in  width  near  the  Seine,  at  Canteleu,  to  300  yards  about  Monville, 
and  decreasing  again  to  thirty  yards  at  Cleres.  The  barometer  fell  sud- 
denly from  29-92  to  2775  inches. 

This  sudden  dilatation  of  the  air  necessarily  upset  the  equilibrium 
of  the  atmosphere  in  the  immediate  neighborhood.  An  inhabitant  of 
Havre  informed  me  that  on  the  day  this  catastrophe  occurred  he  saw  a 
vessel  which  was  three  leagues  off  the  shore  enveloped  in  a  tempest,  al- 
though the  sea  just  outside  Havre  was  relatively  calm. 

The  catastrophe  of  Monville  is  remembered  in  Normandy,  just  as 
a  terrible  shipwreck  is  handed  down  in  the  recollection  of  a  sea-port 
town.  Fortunately  whirlwinds  do  not  often  assume  such  immense  pro- 
portions, or  do  not  occur  at  the  spot  where  large  masses  of  people  are 
congregated.  Several  others,  equally  violent,  perhaps,  have  not  found  any 
element  of  resistance  in  their  path.  That  which  occurred  in  the  neigh- 
borhood of  Troves,  in  1829,  was  in  the  form  of  a  chimney  hanging  from 
a  cloud,  and  emitting  jets  of  flame  and  vapor.  It  soon  changed  to  the 
shape  of  a  serpent,  undulating  above  the  land,  and  leaving  a  track  from 
ten  to  eighteen  paces  broad,  along  a  distance  of  2000  yards,  where  even 
the  grass,  plants,  and  vegetables  growing  upon  the  ground  were  swept 
away.  There  was,  however,  no  loss  of  life  nor  destruction  of  houses. 
That  which  devastated  Chatenay,  near  Paris,  in  June,  1839,  burned  up 
the  trees  that  lay  within  its  circumference,  and  uprooted  those  which 
were  upon  its  line  of  passage.  The  former,  in  fact,  were  found  with 
the  side  which  was  exposed  to  the  storm  completely  scorched  and  burn- 
ed, whereas  the  opposite  side  remained  green  and  fresh.  Thousands  of 
large  trees  were  blown  down  and  lay  all  one  way,  like  wheat-sheaves. 
An  apple-tree  was  carried  over  200  yards  on  to  a  group  of  oaks  and 
elms.  Houses  were  gutted  inside  without  being  blown  down.  Several 
roofs  were  carried  off  as  if  they  were  kites.  An  inside  wall  was  cut 
into  five  nearly  equal  parts  of  eight  yards  each  ;  the  first,  the  third,  and 
the  fifth  were  laid  in  one  direction  ;  the  second  and  the  fourth  in  an  ex- 
actly opposite  direction.  Several  rows  of  slates  had  their  fixings  torn 
out,  without  being  themselves  displaced.  In  a  whirlwind  which  raged 
over  the  village  of  Aubepierre,  in  the  Haute-Marne,  on  the  30th  of 
April,  the  slates  on  the  roof  of  a  wash-house  were  turned  completely 


348  THE  ATMOSPHERE. 

upside  down,  each  rank  being  reversed  as  if  by  the  hand  of  a  work- 
man. 

In  the  sandy  regions  of  the  African  and  Asian  deserts,  the  traveler 


Fig.  64.— Sand  whirlwind. 


sometimes  encounters  gigantic  whirlwinds  of  sand  which  rise  from  the 
earth  to  the  clouds,  twisting  convulsively,  and  emitting  a  sound  like  the 
hissing  of  a  serpent.  This  is  the  phenomenon  represented  in  Fig.  64, 


TROMBES,  OR  WHIRLWINDS.  351 

and  it  is  taken  from  the  travels  of  T.  W.  Atkinson  on  the  frontier-land 
of  Russia  and  China. 

The  water-spouts  that  occur  upon  the  water  differ  only  from  the 
whirlwinds  of  the  air  in  respect  to  their  situation.  In  place  of  dust, 
leaves,  and  other  solid  substances  drawn  up  by  the  whirling  column, 
they  are  composed  of  water,  generally  in  the  form  of  very  condensed 
vapor,  but  sometimes  in  a  liquid  state,  which  becomes  mixed  with  the 
air  of  the  water-spout.  Peltier  cites  many  instances,  extending  over  ev- 
ery degree  of  latitude.  I  can  find  no  case  in  which  they  have  been 
proved  to  have  swallowed  up  a  *vessel.  Generally  the  base  of  the  col- 
umn is  severed  by  discharging  a  cannon  into  it.  Upon  one  occasion, 
however  (in  the  Ionian  Sea.  on  October  29, 1832),  it  appears  that  a  ship 
was  caught  in  a  water-spout  and  tossed  up  and  down,  to  the  great  alarm 
of  the  passengers,  who  were  situated  "like  a  person  at  the  bottom  of  a 
well  who  is  looking  up  into  the  air." 

The  cloud  which  is  attracted  may  descend  near  enough  to  the  ground 
to  raise  up  masses  of  water  as  well  as  floating  substances ;  the  heaviest 
of  these  will  become  detached  from  the  mass  one  by  one  by  reason  of 
their  specific  weight,  but  the  smaller  bodies  may  be  carried  a  great  dis- 
tance, and  may  fall  all  together.  This  is  one  cause  of  showers  of  frogs 
and  fish. 


BOOK  FIFTH. 

WATER-CLOUDS-RAIN. 


THE  WATER  UPON  THE  SURFACE  OF  THE  EARTH.  355 


CHAPTER  I. 

THE  WATER  UPON  THE  SURFACE  OF  THE  EARTH  AND  IN  THE  ATMOS- 
PHERE :  THE  SEA— VOLUME  AND  WEIGHT  OF  THE  WATER  THROUGH- 
OUT THE  GLOBE — PERPETUAL  CIRCULATION  —  VAPOR  OF  WATER  IN 
THE  ATMOSPHERE  —  ITS  VARIATIONS  ACCORDING  TO  THE  HEIGHT, 

THE  LOCALITY,  AND  THE  WEATHER  —  THE   HYGROMETER  —  DEW 

WHITE   FROST. 

THE  globe  to  which  the  force  of  attraction  attaches  us  is  nearly  7958 
miles  in  diameter,  and  is  therefore  about  25,000  miles  in  circumference. 
It  is  a  sphere,  the  cubic  volume  of  which  is  about  264,000,000,000 
cubic  miles.  If  it  consisted  entirely  of  water,  it  would  weigh  about 
1080  trillions  of  tons,  inasmuch  as  water  weighs  about  62£  pounds 
per  cubic  foot.  But,  as  the  earth  is  more  than  five  times  (5'44)  as 
heavy  as  water,  the  weight  of  the  globe  is  about  5880  trillions  of 
tons.  The  atmosphere  which  envelops  our  planet  weighs,  as  I  have 
said,  scarcely  the  millionth  part  of  the  weight  of  the  entire  earth  (the 
T.TT^.TTDU-  part).  Water  occupies  a  not  less  important  part  in  the  ter- 
restrial system  than  air.  The  mean  depth  of  the  oceans  is  about  two 
and  a  half  miles,  taking  into  account  the  uneven  nature  of  their  beds, 
the  level  of  which,  owing  to  the  shores,  table-lands,  valleys,  and  mount- 
ains, varies  from  a  few  yards  to  six  miles. 

Embodied  in  one  mass,  the  water  of  the  sea  (taking  its  average  depth 
at  two  and  a  half  miles)  would  form  a  sphere  900  miles  in  diameter. 
Spread  over  the  whole  spherical  surface  of  the  globe— supposing  the 
surface  to  be  perfectly  even — it  would  cover  it  to  the  depth  of  nearly 
two  miles.  The  density  of  sea- water  is  rather  above  that  of  soft  water : 
its  entire  mass  would  weigh  less  than  the  ^Vrr  Part  of  the  weight  of  the 
earth. 

The  maximum  depth  of  the  ocean  is  about  six  miles,  and  the  part  of 
the  atmosphere  in  which  we  can  breathe  is  of  about  the  same  extent 
It  is  within  this  limited  zone  of  twelve  miles  that  all  the  phenomena  of 
life  take  place,  from  the  submarine  forests  and  strange  animals  which 
inhabit  the  lowest  depths,  to  the  plants  which  vegetate  upon  the  surface 
where  man  has  his  being,  to  the  various  kinds  of  animals  which  live  in 


356  THE  ATMOSPHERE. 

the  open  sky,  to  the  condor  which  soars  above  the  limit  of  perpetual 
snow.  This  zone  of  life  is  very  limited,  when  compared  to  the  size  of  the 
earth,  which  itself  appears  so  small  in  relation  to  the  planetary  system. 

To  form  an  idea  of  the  immense  difference,  we  have  only  to  examine 
an  equatorial  section  of  the  globe.  Even  if  the  sinuosities  are  increased 
fifty-fold,  the  terrestrial  rind  is  almost  a  complete  circle.  The  conti- 
nents and  islands  are  but  the  summits  of  table-lands  and  mountains,  the 
lower  parts  of  which  are  submerged. 

This  water  covers  nearly  three-fourths  of  the  earth,  in  the  state  which 
corresponds  to  the  mean  temperature  of  its  surface — that  is  to  say,  in  a 
liquid  state.  Its  currents  constitute,  as  we  have  seen,  the  grand  arterial 
circulation  of  the  planet.  Not  content  with  thus  prevailing  in  its  ordi- 
nary state,  it  reigns  in  a  solid  state,  in  the  silent  regions  of  the  poles  and 
upon  the  ice-bound  sides  of  inaccessible  mountains ;  and,  in  a  gaseous 
state,  it  reigns  with  more  absolute  sovereignty  in  the  atmosphere,  the 
life  of  which  it  regulates,  and  in  which  it  in  turn  promulgates  abun- 
dance and  dearth,  the  gladness  of  fine  weather,  and  the  gloom  caused  by 
sombre  skies. 

This  water  is  not  motionless,  either  in  the  depth  of  the  oceanic  basin, 
in  the  solid  ice,  or  in  the  atmosphere.  Thanks  to  the  always  active 
power  of  the  sun,  to  the  aerial  currents,  the  water  rises  vertically  from 
the  bed  of  the  sea  to  its  surface,  becomes  vaporized  at  all  temperatures, 
ascends  in  the  shape  of  invisible  vapor  through  the  ocean  of  the  air,  be- 
comes condensed  into  clouds,  travels  across  continents,  falls  again  in  the 
shape  of  rain,  filters  through  the  surface  of  the  soil,  passes  along  the 
strata  of  impermeable  clay,  springs  up  as  a  source  or  fountain-head,  de- 
scends by  the  streamlet  into  the  river,  and  falls  from  the  river  back  into 
the  sea  again. 

Every  source,  every  streamlet,  every  river,  every  stream,  has  its  ori- 
gin in  rain.  Even  the  mineral  waters  are  produced  by  the  same  cause, 
and  their  heat  is  merely  due  to  the  profound  depths  from  which  these 
meteoric  waters  have  been  brought  up ;  and  they,  moreover,  continue 
to  ascend  through  the  interstices  of  the  rocks,  afterward  returning  to 
the  level  of  their  primitive  reservoir,  like  a  siphon.  The  sun,  as  it 
evaporates  the  sea-water,  leaves  behind  it  the  salt,  which  is  not  volatile. 
That  is  why  rain-water  is  soft,  and  that  of  a  running  stream  also.  The 
salt  never  leaves  the  sea,  and  its  quantity  is  such  that  it  would  cover 
the  whole  surface  of  the  globe  to  a  depth  of  ten  yards. 

Just  as  the  blue  color  of  the  sky  is  due  to  the  vapor  of  water,  so  also 


THE  WATER  UPON  THE  SURFACE  OF  THE  EARTH.  357 

is  the  color  of  water  itself,  taken  in  a  mass,  blue ;  its  shades  vary  from 
that  hue  to  green,  according  to  the  action  of  light.  The  vapor  of  water 
mixed  with  the  air  is  of  the  highest  importance  in  the  distribution  of 
temperatures ;  both  its  formation,  as  well  as  its  movement  from  place  to 
place,  represent  a  formidable  force  which  is  permanently  in  action.  The 
air  can  contain  more  vapor  of  water  in  proportion  as  it  is  heated.  A 
given  diminution  of  temperature  brings  it  to  its  saturating  limit,  with- 
out in  any  way  adding  to  the  quantity  of  vapor  which  it  contains.  To 
ascertain  the  quantity  of  vapor  of  water  mixed  with  the  air  at  a  given 
moment,  a  thermometer,  for  instance,  suspended  in  the  air  might  be 

made  gradually  colder  until  it  indicated  the  limit  of  saturation that  is, 

until  its  bulb  was  covered  with  condensed  vapor,  or  dew.  By  ascer- 
taining what  quantity  of  vapor  of  water  corresponds  to  this  thermo- 
metrical  degree  of  saturation,  we  should  learn  the  real  quantity  suspend- 
ed in  the  air  at  the  moment  of  the  experiment 

The  instruments  for  measuring  the  moisture  of  the  air  have  received 
the  name  of  hygrometer  (w-y/ooe>  moist,  /wrpov,  measure).  That  in  most 
general  use  is  formed  of  two  thermometers  exactly  alike,*  and  placed  side 
by  side.  The  bulb  of  one  is  enveloped  by  a  piece  of  muslin,  which  is 
kept  constantly  moist.  The  moistened  thermometer  has  a  lower  tem- 
perature in  consequence  of  evaporation  proceeding  from  the  moist  mus- 
lin. The  difference  of  reading  between  the  two  thermometers  is,  there- 
fore, dependent  upon  the  more  or  less  moisture  in  the  air.  The  hy- 
grometrical  state  of  the  atmosphere  is  not  the  same  from  the  top  to  the 
bottom,  like  the  proportions  of  oxygen  and  nitrogen.  As  a  rule,  it  in- 
creases, beginning  from  the  surface  of  the  ground  up  to  a  certain  height, 
where  there  exists  a  zone  of  maximum  moisture ;  above  that  point  it 
decreases.  I  will  not  venture  to  trace  a  diagram  of  this  variation  in  the 
moisture  according  to  the  height,  as  I  have  done  in  regard  to  the  de- 
crease of  atmospheric  pressure  and  of  the  temperature,  for  the  observa- 
tions which  I  have  made  upon  this  head  are  wanting  in  number  and  in 
precision.  Those  taken  by  Glaisher  are  much  more  precise,  and  have 
been  made  by  different  hygrometers.  They  show  that,  generally  speak- 
ing, the  moisture  increases  from  the  surface  of  the  soil  to  a  height  of 
3500  feet,  and  that  after  that  it  diminishes,  there  being,  however,  spaces 
which  represent  moist  strata  of  air  of  varying  thicknesa 

The  observations  taken  upon  mountains  confirm  the  increase  which 
*  See  "  Gkisher's  Hygrometrical  Tables  "  for  description  and  use  of  dry  and  wet  bulb  ther- 
mometers. — Ei>. 


358 


THE  ATMOSPHERE. 


was  first  noticed  to  be  in  proportion  to  elevation.  Kaemtz  has  ascer- 
tained the  degree  of  humidity  to  be  a  mean  of  84  upon  Mount  Righi 
as  against  74  at  Zurich  beneath  it.  Bravais  and  Martins  registered  76 
upon  the  summit  of  the  Faulhorn,  when  at  Milan  there  was  only  63. 
At  heights  exceeding  3300  feet  the  moisture  decreases,  in  spite  of  the 
special  augmentations  due  here  and  there  to  currents  which  lie  one  over 
the  other. 

Upon  the  surface  of  the  ground,  the  relative  moisture  of  the  air  va- 
ries according  to  the  time  of  day,  in  inverse  ratio  to  the  temperature. 
The  warmer  the  air,  the  drier  it  will  be ;  the  colder  it  is,  the  more  read- 
ily will  it  be  saturated  with  moisture.  In  our  temperate  regions,  the 
hygrometrical  state  of  the  air  augments,  with  little  fluctuation,  toward 
sunrise  during  the  minimum  of  temperature ;  afterward  falls,  until  about 
2  P.M.,  at  the  maximum  of  heat;  and  rises  again  toward  evening  and  at 
night.  Twenty  years  of  daily  repeated  observations  (1843-1863),  taken 
at  Brussels  by  the  aid  of  the  Saussure  hygrometer  and  dry  and  wet 
bulb  thermometers,  have  furnished  M.  Quetelet  with  the  information 
that  the  mean  degree  of  humidity  at  noon  is  as  follows: 


February  

84 

August  

68 

March  

73 

September  

74 

April       

66 

October  

80 

Mav 

65 

85 

64 

December  

89 

Where  complete  saturati 

3n  is  represented  by  100. 

We  see  that  the  maximum  of  relative  humidity  occurs  in  December, 
and  the  minimum  in  June.  This  invisible  atmospheric  moisture,  the 
presence  of  which  is  only  revealed  by  aid  of  delicate  instruments,  con- 
fers upon  the  landscape  all  the  variety  with  which  it  is  endowed — the 
emerald  green  of  the  Irish  pastures,  the  blue  sky  of  the  Mediterranean, 
the  splendor  of  tropical  vegetation — and  it  becomes  visible  in  the  shape 
of  dew  as  soon  as  a  diminution  of  temperature  brings  it  to  the  point 
of  saturation.  If  it  is  the  air  itself  which  becomes  colder,  it  is  made 
opaque  by  the  passage  of  the  vapor  in  a  liquid  state,  and  hence  arises 
tog.  If  it  be  a  solid  body  which  is  thus  rendered  cold,  the  moisture 
becomes  condensed  upon  its  surface,  and  the  result  is  dew. 

Dew  does  not  come  down  from  the  sky,  as  is  still  taught  in  the 
French  primary  schools.  Its  production  is  in  no  degree  assimilated  to 
that  of  rain.  It  is  formed  at  the  spot  where  it  is  seen.  If  small  por- 


DEW  AND  RADIATION.  359 

tions  of  grass,  cotton,  or  other  fibrous  substance  be  exposed  to  the  sky 
on  a  fine  night,  it  is  found  that,  after  a  certain  time,  their  temperatures 
are  fifteen,  eighteen,  and  even  twenty  degrees  below  that  of  the  cir- 
cumambient atmosphere. 

In  places  where  the  sunlight  does  not  penetrate,  and  whence  a  large 
extent  of  sky  can  be  seen,  this  difference  between  the  temperature  of 
the  grass,  cotton,  wool,  etc.,  and  the  atmosphere  is  noticeable  between  3 
and  4  P.M. — that  is  to  say,  as  soon  as  the  temperature  diminishes;  in 
the  morning  it  continues  for  several  hours  after  sunrise. 

The  observations  of  Wells,  continued  by  Arago,*  have  proved  that 
on  a  clear  night  the  grass  of  a  meadow  may  be  ten  to  twenty  degrees 
colder  than  the  air ;  if  the  weather  becomes  cloudy,  the  grass  at  once 
increases  several  degrees  in  temperature,  without  any  increase  in  that 
of  the  atmosphere. 

This  diminution  of  heat  is  due  to  nocturnal  radiation.  When  there 
is  nothing  to  prevent  the  heat  of  a  body  from  becoming  dispersed,  it 
gradually  becomes  irradiated  and  lost.  The  transparent  air  does  not 
suffice  to  prevent  this  loss  of  heat.  But  a  cloud,  a  wooden  screen,  a 
sheet  of  paper,  a  little  smoke  even,  will  answer  the  purpose.  Without 
obstacles  of  some  kind,  the  substance  becomes  colder  according  to  its 
power  of  radiation,  which  is  itself  dependent  upon  the  nature  of  the 
substance  (it  is,  for  instance,  very  great  in  the  case  of  glass,  and  very 
trifling  with  metals) ;  and  when  the  temperature  of  the  body  thus  ex- 
posed has  reached  that  of  the  point  of  saturation,  the  atmospheric  mois- 
ture is  deposited  upon  it,  taking  at  first  the  shape  of  spheroidal  drops ; 
then,  when  these  drops  are  sufficiently  weighty  and  near  together,  they 
extend  like  a  shallow  pool  of  water  over  the  surface  of  the  substance. 

Dew  is  never  abundant  except  when  the  nights  are  calm  and  bright. 
A  little  dew  may  be  seen  when  the  nights  are  cloudy,  if  there  be  no 
wind,  or  even  with  wind  if  the  weather  is  bright;  but  there  is  never 
any  sign  of  it  when  there  is  wind,  and  the  sky  is  cloudy  as  well.  The 
circumstances  which  favor  an  abundant  deposition  of  dew  more  gener- 
ally occur  in  spring-time  and  in  autumn — the  latter  especially — than  in 
summer.  It  must  be  remembered,  too,  in  addition  to  the  above  fact, 
that  the  differences  between  the  temperature  of  day  and  night  are  never 
greater  than  they  are  in  spring  and  autumn. 

The  phenomenon  of  the  deposit  of  dew  upon  a  dense  and  smooth 

*  [And  by  the  Editor.  See  "Phil.  Trans., "part  ii.,  1847,  for  paper  on  "Radiation  at 
Night  from  the  Earth,  and  several  Substances  placed  on  or  near  it."] 


360  THE  ATMOSPHERE. 

substance — upon  a  sheet  of  glass,  for  instance — resembles  that  seen 
when  a  pane  of  glass  is  exposed  to  a  current  of  vapor  of  water  warmer 
than  itself:  first,  a  light  and  uniform  layer  of  moisture  dims  the  sur- 
face; then  are  formed  irregular  and  flat  drops,  which  run  together  af- 
ter they  have  acquired  a  certain  volume,  and  flow  in  all  directions. 

This  may  be  seen  whenever  any  substance  which  has  been  rendered 
cold  by  exposure  to  a  low  temperature  is  taken  into  a  warm  room ;  the 
substance  at  once  becomes  covered  with  moisture.  In  the  same  way 
glass  placed  in  a  room  where  a  large  number  of 'persons  are  dining  is 
at  once  dimmed  by  the  thick  stratum  of  dew  which  the  invisible  vapor 
mixed  with  the  surrounding  air  deposits.  The  glasses  of  a  pair  of 
spectacles  which  have  been  exposed  to  cold  air  will  often  be  found  ob- 
scured in  the  same  way. 

If,  during  a  frost,  the  windows  of  a  room  in  which  a  large  company 
has  been  dining  are  suddenly  thrown  open,  a  cloud  forms  instantane- 
ously in  the  path  of  the  cold  air,  and  the  ceiling  is  made  damp  by  a 
long  stain  of  condensed  vapor. 

Dew  is  a  phenomenon  of  importance,  not  only  because  of  the  absolute 
quantity  which  any  one  point  of  the  globe  receives,  but  because  of  the 
extent  of  ground  over  which  it  may  be  deposited.  It  is  mostly  in 
tropical  regions  that  its  effects  upon  vegetation  are  the  most  marked 
and  the  most  favorable.  When  the  air,  nearly  saturated  with  vapor  at 
the  temperature  of  86°,  contains  more  than  thirteen  grains  of  water  to 
the  cubic  foot,  the  water  falls  abundantly  during  the  declining  tempera- 
ture of  night ;  it  makes  the  leaves  drip,  and  in  the  morning  grass  is 
as  wet  as  if  there  had  been  heavy  rain.  The  dew  is  known  to  deposit 
in  greater  or  in  lesser  quantities,  but  it  has  not  been  found  possible  to 
measure  it,  because  it  does  not  fall  like  rain ;  its  appearance  depends 
upon  the  radiating  power  of  the  body  which  it  moistens,  for  it  is  only 
deposited  upon  substances  which  are  colder  than  the  surrounding  air, 
and  in  increased  quantity  according  as  the  difference  of  temperature  is 
greater.  Plowed  land,  fallow,  forests,  rocks,  and  sand  vary  much  in  re- 
spect to  the  dew  which  deposits  upon  them ;  and,  more  than  that,  the 
leaves  of  all  plants  do  not  possess  an  equal  radiating  power,  and  the 
intensity  of  their  diminution  of  temperature,  influencing  the  deposit  of 
dew  which  ensues  upon  it,  is  dependent  upon  their  distance  from  the 
ground,  their  color,  the  smoothness  or  the  ruggedness  of  their  epidermis. 
The  dew  alights  upon  the  leaves  of  mangel-wurzel,  while  the  tops  of 
potatoes  in  an  adjoining  field  will  hardly  be  moist. 


DEW  AXD  RADIATION.  ogi 

M.  Boussingault  has  endeavored  to  measure  these  quantities  of  dew. 
After  certain  nights,  when  the  dew  had  fallen  abundantly,  he  used  to 
repair  to  the  meadows  on  the  banks  of  the  Sailer  before  sunrise ;  there 
by  aid  of  a  sponge,  he  soaked  up  the  water  over  forty-three  square  feet 
of  glass,  and  this  he  placed  in  a  bottle  and  weighed.  In  some  instances 
it  was  found  to  exceed  two  pounds  in  weight. 

Dew  and  mist  contain  about  the  same  proportions  of  ammonia  and 
nitric  acid ;  both,  moreover,  have  a  great  analogy  to  rain  when  it  be- 
gins to  fall— when  it  is,  so  to  speak,  in  process  of  washing  the  air.  It 
is,  in  fact,  in  the  first  part  of  a  shower  of  rain  after  a  season  of  long 
drought,  that  there  is  present  the  greatest  amount  of  carbonic  acid,  car- 
bonate and  nitrate  of  ammonia,  organic  matters,  and  dust  of  every  kind. 
If  a  close  examination  be  made  of  the  substances  which  air  contains  in 
infinitesimally  small  quantities,  it  is  in  the  mist,  the  dew,  the  first  drops 
of  rain,  the  first  flakes  of  snow  and  hail,  that  we  must  look  for  them. 

White  frost,  which  is  so  fatal  to  vegetation  in  spring,  and  which  has 
given  such  a  bad  reputation  to  the  harvest-moon,  is,  in  reality,  the  dew 
frozen  by  the  same  cause  as  that  which  led  to  its  formation — nocturnal 
radiation. 

In  1871,  A.  Wilson,  having  followed  the  movements  of  a  thermom- 
eter during  a  winter  night  when  the  weather  alternated  constantly  be- 
tween clear  and  foggy,  found  that  it  always  rose  about  a  degree  at  the 
same  moment  that  the  atmosphere  clouded  over,  and  fell  to  the  point 
at  which  it  had  previously  stood  when  the  mist  cleared  off.  His  son, 
Patrick  Wilson,  asserts  that  the  instantaneous  effect  of  a  thermometer 
hung  up  in  the  open  air  is  to  cause  an  elevation  of  5°.  The  researches 
of  Pictet,  undertaken  in  1777,  and  published  in  1792,  coincide  nearly 
with  the  above. 

It  is  a  curious  circumstance,  which  was  discovered  by  Pictet,  that, 
when  the  nights  are  still  and  clear,  the  temperature  of  the  air,  instead 
of  diminishing  the  higher  from  the  ground,  shows,  on  the  contrary,  a 
progressive  rate  of  increase,  at  least  up  to  a  certain  height.  A  ther- 
mometer nine  feet  above  the  soil  marked  throughout  the  night  4£° 
Fahr.  less  than  an  exactly  similar  instrument  which  was  attached  to 
the  summit  of  a  pole  fifty  feet  high.  About  two  hours  after  sunrise, 
and  two  hours  before  sunset,  the  two  instruments  were  exactly  the 
same.  Toward  noon  the  thermometer  nearest  the  ground  was  often 
4£°  higher  than  the  other.  When  the  sky  was  covered  with  clouds, 
the  two  instruments  corresponded  exactly,  both  by  day  and  night. 


362  THE  ATMOSPHERE. 

These  observations  have  been  confirmed.  Wells  having  placed  at 
the  four  corners  of  a  square  four  small  pegs,  which  stood  perpendicu- 
larly four  inches  above  the  surface  of  a  meadow,  spread  over  them 
horizontally  a  fine  cambric  handkerchief,  and  during  five  nights  com- 
pared the  temperature  of  the  small  square  of  grass  covered  by  it  with 
the  surrounding  portion  which  remained  fully  exposed  to  the  air.  The 
turf  that  was  protected  from  radiation  by  the  handkerchief  was  at  times 
11°  warmer  than  the  other.  While  the  latter  was  completely  frozen, 
the  temperature  of  the  turf  protected  from  the  air  was  several  degrees 
above  32°.  With  the  sky  very  cloudy,  a  screen  of  cambric,  matting, 
or  any  other  substance,  produces  scarcely  any  effect. 

Mr.  Glaisher  finds,  after  three  years'  consecutive  observations  at 
Greenwich,  that  the  temperature  of  the  air  twenty-two  feet  above  the 
ground  is  higher  than  that  at  four  feet  at  every  hour  of  the  night  and 
day  during  the  months  of  November,  December,  January,  and  Febru- 
ary ;  that  it  is  higher  at  night  and  in  the  evening  in  May,  June,  and 
July ;  and  that  it  is  also  higher  during  night-time  and  in  the  afternoon 
in  March,  April,  August,  September,  and  October.  At  an  elevation 
of  fifty  feet  the  temperature  is  also  higher  during  the  night  throughout 
the  whole  year.  With  the  sky  cloudy,  the  temperature  remains  the 
same. 

In  June  of  1871  the  attention  of  the  Academy  of  Sciences  was  di- 
rected to  the  subject  of  late  frosts  by  M.  Ste.  Claire-Deville  and  M.  Elie 
de  Beaumont.  The  immediate  instance  in  hand  was  the  frost  which 
occurred  on  the  18th  of  May  (Ascension-day)  and  extended  to  the 
vines  and  their  crops  around  Paris  and  in  the  centre  of  France.  As  I 
myself  had  seen  a  vine  which  had  been  frozen  in  the  Haute-Marne,  I 
showed  by  a  few  comparisons  that  this  disastrous  frost  extended  over 
quite  oneOialf  of  France  at  the  same  •moment.  It  would  certainly  be 
most  desirable  to  find  some  means  for  protecting  crops  during  the  criti- 
cal period  which  follows  the  blossoming,  as  many  severe  losses  would 
thus  be  prevented. 


THE  CLOUDS. 


CHAPTER  II. 

THE  CLOUDS :   WHAT  A  CLOUD  IS — THE  MANNER  OF  ITS  FORMATION 

MIST — OBSERVATIONS   TAKEN  FROM  A  BALLOON  AND   FROM  MOUNT- 
AINS—  DIFFERENT     KINDS     OF     CLOUDS  —  THEIR     SHAPES THEIR 

HEIGHTS. 

THE  invisible  vapor  of  water  spread  throughout  the  atmosphere,  the 
distribution  and  variations  in  which  I  have  just  pointed  out,  becomes 
visible  when  a  decline  in  the  temperature  or  an  addition  of  moisture 
brings  it  to  the  point  of  saturation.  Suppose,  for  instance,  that  a  cer- 
tain quantity  of  air  at  eighty-six  degrees  contains  478  grains  of  vapor 
of  water,  this  air  will  be  quite  transparent.  If  by  some  cause  or  other 
this  air  descends  to  seventy-seven  degrees,  or  receives  an  accession  of 
moisture,  it  will  become  opaque.  A  diminution  of  nine  degrees  of 
heat  will  cause  108  grains  of  vapor  of  water  to  be  condensed  and  to 
become  visible.  That  is  what  a  cloud  really  is :  vapor  of  water  which 
the  air,  being  saturated,  is  no  longer  able  to  absorb,  and  which  becomes 
separated  from  it  by  passing  into  the  state  of  small  vesicles. 

This  passage  from  the  gaseous  to  the  liquid  state  takes  place  indif- 
ferently at  all  elevations.  When  it  occurs  at  the  level  of  the  soil,  it  is 
termed  mist.  But  there  is  no  essential  difference  between  a  cloud  and 
mist.  Traveling  through  the  clouds  in  a  balloon,  meeting  no  resist- 
ance, the  air  is  simply  more  or  less  opaque,  more  or  less  cold,  more  or 
less  damp,  just  as  is  the  case  upon  the  surface  of  the  ground,  according 
to  the  diversity  of  the  mists.  This  is  also  the  case  with  the  clouds 
when  one  is  enveloped  in  them  upon  the  summits  of  a  mountain. 

Though  there  is  no  essential  difference  between  mists  and  clouds,  there 
is,  however,  one  in  fact,  viz.,  that  a  mist  is  vapor  of  water  passing  from 
the  visible  to  the  invisible  state ;  whereas  a  cloud  is  a  grouping  of  visi- 
ble vapors  in  some  given  shape.  The  first  is  motionless,  the  second  is 
endowed  with  movement.  Let  us  consider  the  mist  first. 

Seen  through  a  glass,  mist  is  composed  of  small  and  opaque  bodies. 
A  closer  study  shows  that  these  small  bodies  are  composed  of  water, 
obeying  the  laws  of  universal  gravitation.  The  molecules  of  water  are 
grouped  together  in  the  form  of  spherules.  Are  these  spherules  full 


364  THE  ATMOSPHERE. 

or  hollow?  Such  is  the  question  upon  which  meteorologists  are 
divided.  The  opinion  already  given  by  Halley  that  these  spherules 
are  hollow,  and  that  the  water  is  but  an  envelope,  seems  the  best 
founded. 

Take  a  cupful  of  some  dark-colored  liquid,  such  as  coffee  or  China 
ink,  dissolved  in  water;  warm  it,*and  place  it  in  the  sun's  rays:  if  the 
air  be  still,  the  vapor  will  ascend  and  soon  disappear;  if  looked  at 
through  the  glass,  it  will  be  seen  that  globules  are  rising.  The  small- 
est run  rapidly  over  the  surface  of  the  magnifying-glass,  the  others  fall 
back  on  to  the  liquid  mass.  Saussure  adds  that  the  small  vesicles 
which  rise  differ  so  much  from  those  which  fall  back  that  it  is  impossi- 
ble to  doubt  that  the  first  are  hollow. 

The  way  in  which  they  act  when  exposed  to  the  light  is  also  favor- 
able to  this  supposition,  for  they  do  not  scintillate  like  the  full  drops 
when  they  are  exposed  to  a  bright  light.  Every  one  must  have  re- 
marked that  soap-bubbles  are  generally  very  brilliant  in  color.  The 
same  must  also  have  been  noticed  with  bubbles  from  other  viscous  sub- 
stances, and  it  is  the  easier  to  observe  them  because  they  continue  a 
longer  time.  These  colors  rise  from  the  division  of  the  incident  rays 
into  two  parts.  Some  of  the  rays  are  reflected  by  the  outside  surface ; 
others  penetrate  through,  and  are  reflected  by,  the  inner  surface.  The 
envelope  of  the  sphere  must  be  thin,  to  admit  of  this  taking  place. 
Kratzenstein  having  examined  in  the  sun  and  through  a  magnifying- 
glass  the  vesicles  which  ascend  out  of  hot  water,  observed  upon  their 
surface  colored  rings  like  those  of  soap-bubbles;  and  not  only  was  he 
convinced  that  their  structure  is  analogous  to  that  of  soap-bubbles, 
but  he  was  further  successful  in  calculating  the  thickness  of  their  en- 
velope. 

De  Saussure  and  Kratzenstein  attempted  to  measure  by  aid  of  the 
microscope  the  diameter  of  the  vesicles  which  compose  the  vapor  of 
water.  But  it  is  difficult  to  arrive  at  any  positive  result,  for  it  is  the 
vesicles  rising  from  mist,  and  not  those  from  hot  water,  which  it  is  nec- 
essary to  measure.  Fortunately,  some  of  the  optical  phenomena  which 
occur,  when  the  sun  shines  through  clouds,  furnish  us  with  a  means  of 
arriving  at  this  result. 

Kaemtz  has  taken  a  great  number  of  measurements  in  Central  Ger- 
many and  Switzerland;  he  has  ascertained  that  upon  an  average  the 
diameter  of  the  vesicles  of  mist  is  about  '00087  of  an  inch,  and  that  it 
varies  in  the  different  seasons  as  follows: 


CLOUDS,  MIST,  AND  FOG. 


365 


DIAMETER  OF  THE  VESICLES  OF  THE  MIST. 


Inch. 

Inch. 

January  

0-0106 

July..  . 

February  

0-0138 

August  

0*0055 

March               

0-0079 

April                  

0-0075 

October  . 

May 

0-0059 

June  

0-0071 

December  

0-0134 

It  will  be  seen  that  there  is  an  almost  regular  progression  from  win- 
ter till  summer ;  the  anomalies  arise  from  the  small  number  of  obser- 
vations that  have  been  taken.  Thus  in  winter,  when  the  air  is  very 
moist,  the  diameter  of  the  vesicles  is  twice  as  great  as  in  summer,  when 
the  air  is  dry ;  but  this  diameter  also  varies  in  the  course  of  a  single 
month.  It  attains  its  minimum  when  the  weather  is  very  fine;  it  in- 
creases when  there  are  signs  of  rain  ;  and  before  the  fall  it  varies  con- 
siderably in  the  same  cloud,  which  probably  contains  a  large  number 
of  drops  of  water  mixed  with  vesicular  vapor. 

Autumn,  like  spring,  is  the  season  of  abundant  dew.  The  cooling 
process  to  which  the  ground  is  subject,  when  the  nights  are  clear,  and 
the  moisture  of  the  air  nearer  precipitation  than  in  summer,  causes  the 
atmospheric  water  to  be  deposited  upon  terrestrial  objects  which  have 
diminished  in  temperature,  just  as  in  a  crowded  room  the  moisture  of 
the  heated  air  affects  the  glass  brought  in  from  outside.  The  steam  of 
hot  dishes,  the  breath  of  the  persons  present,  the  combustion  of  the 
lights,  make  the  air  of  the  dining-room  hot  and  moist,  and  cause  water 
to  trickle  down  the  vases  containing  ice.  In  autumn,  the  nocturnal 
coldness  of  the  ground  often  communicates  itself  to  the  stratum  of  air 
immediately  above,  and  hence  arise  the  low  fogs  which  are  soon  dissi- 
pated by  the  sun's  rays.  If  the  ground  be  uneven,  the  cold  air  of  fogs 
descends  into  the  valleys,  and  seems,  to  any  one  standing  upon  an  emi- 
nence, a  white  sea  perfectly  level.  As  a  child,  I  have  often  watched 
before  sunrise,  from  the  ramparts  of  Langres,  the  ocean  of  grayish  va- 
pors that  extend  through  the  valley  of  the  Marne,  and  the  waves  of 
which  reached  to  within  a  few  feet  of  where  I  was  standing.  The 
height  of  the  ramparts  at  Langres  is  near  1500  feet  above  the  level  of 
the  sea.  In  winter  the  view  sometimes  extends  at  sunrise  so  far  be- 
yond the  mist  in  the  plain,  that  the  white  outline  of  Mont  Blanc  is  dis- 
cernible with  the  naked  eye. 

To  witness  a  spectacle  of  this  kind  at  its  best,  it  is  necessary  to  bo 


366  THE  ATMOSPHERE. 

upon  the  top  of  a  lofty  mountain,  whence  the  view  embraces  a  vast 
horizon,  and  at  sunrise  after  a  day  when  the  clouds  have  obscured  the 
sky  of  the  country  below.  The  clouds,  disturbed  in  a  thousand  ways 
by  the  rays  of  the  sun  and  the  light  winds  which  are  the  natural  con- 
sequence, are  not  very  level  during  the  day-time.  But  at  night  the 
equilibrium  and  the  level  are  restored,  and  a  sea  of  aerial  vapors  ex- 
tends far  as  the  eye  can  reach  beneath  the  feet  of  the  observer.  The 
elevated  summits  of  the  isolated  mountains  around  him  break  here  and 
there  through  the  nebulous  ocean,  above  which  soars  from  time  to  time 
an  eagle  in  quest  of  its  prey.  Standing  in  the  valley,  in  the  midst  of 
the  mist,  the  sun's  rays,  as  they  play  through  the  foliage,  delineate  brill- 
iant beams  of  light,  the  ensemble  of  which  forms  what  is  called  a  glory 
not  more  than  a  few  yards  above  the  head  of  the  spectator.  This  gk- 
ry,  which  emanates  from  the  tree  immersed  in  the  fog,  recalls  to  mind 
Moses's  burning  bush. 

Sometimes  only  the  surface  of  rivers  is  covered  with  fog,  because 
water  emits  vapor  which  becomes  condensed  in  the  air  which  lies  over 
them,  and  which  becomes  cold  after  sunset.  The  air  takes  almost  in- 
stantaneously the  temperature  of  the  bodies  to  which  it  is  in  contigu- 
ity. During  a  calm  and  clear  night,  the  portion  of  the  atmosphere 
which  lies  over  water  will  be  warmer  than  that  above  dry  land. 

In  calm  weather,  where  water  is  abundant,  the  lower  strata  of  the 
atmosphere  become  laden  with  the  extreme  amount  of  moisture  com- 
patible with  their  temperatures.  I  have  already  stated  that  the  mois- 
ture which  the  air  contains  when  it  is  saturated  is  of  a  fixed  quantity, 
which  varies  according  to  its  temperature.  If  saturated  air  becomes 
cold  by  contact  with  a  solid  body,  it  deposits  upon  the  surface  of  that 
body  a  portion  of  its  moisture ;  but  when  the  cooling  process  takes 
place  in  the  very  midst  of  the  gaseous  mass,  the  moisture  that  is  set 
free  passes  off  in  small  floating  vesicles,  which  affect  its  transparency : 
it  is  these  vesicles  which  constitute  clouds  and  mists. 

Let  us  suppose  that  some  circumstance — a  small  declivity  of  the  soil, 
for  instance,  a  slight  puff  of  wind — causes  a  fusion  to  take  place  at 
night  between  the  air  that  lies  over  a  river  or  sea,  and  that  which  is 
above  the  land :  the  latter,  which  is  colder,  diminishes  the  temperature 
of  the  former ;  the  former  also  loses  a  part  of  the  humidity  which  it 
contained,  and  which  did  not  at  first  cause  any  alteration  in  its  diapha- 
nous condition.  But  as  this  moisture  gradually  resolves  itself  into  a 
state  of  vesicular  vapor,  the  air  becomes  thick ;  when  the  number  of 


CLOUDS,  MIST,  AND  FOG. 


367 


floating  vesicles  becomes  very  large,  a  heavy  fog  comes  on.  The  dis- 
tribution of  fog  throughout  the  year  corresponds  with  that  of  humidi- 
ty and  temperature.  Fogs  are  much  more  numerous  in  winter  than 
in  summer.  The  Brussels  Observatory,  which  has  recorded  them  with 
great  care,  gives  the  following  as  the  number  of  days  on  which  there 
have  been  fogs  for  the  last  thirty  years  (1833-1863) : 


January 259 

February 168 

March 138 

April 62 


May 

June 

July 


August 76 

September 159 

October 228 

November 276 

December . 315 

Total 1822 


Under  certain  circumstances  the  fog  is  very  thick,  and  is  bounded  by 
a  plane  surface  like  a  sheet  of  water,  rising  slowly  in  the  still  air,  and 
enveloping  all  surrounding  objects  with  a  cold  and  damp  embrace. 
M.  Raynal,  whose  vessel  was  wrecked  off  Auckland  Island  in  1864, 
was  witness  of  a  curious  instance  of  fog,  which  he  relates  in  this  way : 
Having,  on  the  9th  of  August,  climbed  one  of  the  mountains  in  the 
island,  he  was  making  his  way  down  again  with  one  of  his  compan- 
ions, following  a  narrow  path  between  two  precipices.  "  I  was  una- 
ble," he  says,  "  to  move  a  step,  for  we  could  not  see  where  to  put  our 
feet.  We  passed  at  least  an  hour  in  this  way,  absolutely  motionless, 
and  holding  each  other  by  the  hand,  while  the  cold  began  to  benumb 
our  limbs.  Fortunately  a  breeze  sprang  up,  and  dividing  the  fog  into 
two  parts,  gradually  carried  it  away." 

But  it  is  in  the  frozen  latitudes  that  the  fogs  are  thickest.  At  Spitz- 
bergen,  says  M.  Martins,  the  mists  are  almost  continuous,  and  so  thick 
that  it  is  impossible  to  make  out  objects  which  are  a  few  paces  off. 
These  damp,  cold,  and  piercing  mists  often  wet  as  much  as  rain.  Thun- 
der-storms are  unknown  in  these  regions,  even  during  summer.  To- 
ward autumn  the  fogs  increase,  rain  changes  into  snow.  Fig.  67,  illus- 
trative of  an  incident  during  the  scientific  voyage  to  which  I  have  re- 
ferred, gives  an  idea  of  these  immense  and  perpetual  fogs. 

In  countries  where  the  soil  is  damp  and  hot,  and  the  air  damp  and 
cold,  thick  and  frequently  recurring  fogs  must  be  expected ;  this  is  the 
case  in  England,  the  shores  of  which  are  surrounded  by  seas  with  a  high 
temperature.  It  is  the  same  with  the  polar  seas  and  Newfoundland, 


368  THE  ATMOSPHERE. 

where  the  Gulf  Stream,  which  comes  from  the  south,  has  a  higher  tem- 
perature than  that  of  the  air. 


Fig.  66. — Intense  fog  in  one  of  the  islands  of  the  Antipodes. 

In  London  the  fogs  are  at  times  dense.  Every  year  the  journals  re- 
cord that  it  has  been  found  necessary  to  light  gas  in  the  middle  of  the 
day,  both  in  the  streets  and  houses.  Very  heavy  fogs*  also  occur  in 

*  There  are  at  times  dry  fogs.  They  have  no  connection  with  the  hygrometrical  states  I 
am  now  discussing.  They  are  generally  due  to  the  smoke  of  burning  prairies,  and  may  ex- 


CLOUDS,  MIST,  AND  POO.  ggg 

Paris  and  Amsterdam,  the  sky,  at  a  short  distance  from  these  cities  be- 
ing  at  the  same  moment  perfectly  clear. 


Pig.  67.— Intense  fog  in  the  Spitzbergen  Mountains. 

Thick  fogs  emit,  too,  a  noxious  odor  when  they  become  impregnated 

tend  over  a  vast  distance.  The  smoke  of  the  heath  in  Holland  sometimes  reaches  as  far  as 
Austria,  hundreds  of  leagues  off.  The  smoke  of  volcanoes  also  extends  very  far,  that  from 
Honolulu  having  been  seen  in  1868  at  a  distance  of  200  miles  from  the  mouth  of  the  volcano. 
In  1865  the  smoke  from  a  great  fire  at  Limoges  covered  the  sky  seventy-five  miles  off.  The 
most  intense  dry  fog  known  is  one  that  occurred  in  1783. 

24 


370  THE  ATMOSPHERE. 

with  the  different  exhalations  which  may  find  their  way  into  the  lower 
strata  of  the  atmosphere.  Ammonia  may  often  be  discovered.  It  is 
not  rare  to  find  it  accompanied  by  a  smell  of  peat  in  Belgium  and  the 
north  of  France.  During  the  cold  and  damp  fogs  of 'the  month  of  Oc- 
tober, 1871,  in  Paris,  the  smell,  of  petroleum  was  several  times  per- 
ceptible. 

If  a  chain  of  mountains  be  looked  at  from  a  distance,  it  will  often  be 
seen  that  a  cloud  hangs  over  each  peak,  but  that  the  intervals  between 
them  are  clear.  This  state  of  things  may  last  for  hours  and  even  days, 
but  this  absence  of  motion  is  only  apparent,  for  there  is  frequently  a 
strong  wind  blowing  over  these  summits,  which  condenses  the  vapor  as 
it  ascends  the  flanks  of  the  mountains.  As  soon  as  it  disappears  from 
the  summits,  the  wind  also  vanishes.  In  Alpine  passes,  the  formation, 
the  movements,  and  the  disappearance  of  clouds  form  a  spectacle  of 
very  varied  beauty. 

The  clouds  which  ascend  the  mountain  side  of  a  day-time,  by  virtue 
of  the  diurnal  ascending  currents,  often  dissolve  when  they  reach  the 
summits  under  the  influence  of  an  upper  wind,  which  is  comparatively 
dry  and  warm.  It  is  of  an  evening  especially  that  this  is  the  most  no- 
ticeable, and  the  phenomenon  generally  occurs  upon  the  ridges  and 
summits  of  the  passes  which  lead  to  them.  The  fog  then  seems  to  make 
its  way  in  the  direction  from  which  the  wind  is  blowing,  yet,  notwith- 
standing the  surface  by  which  it  is  bounded,  remains  stationary. 

Very  often,  sombre  clouds,  passing  rapidly  over  the  StGothard  Hos- 
pice, are  precipitated  in  vast  masses  into  the  deep  gorge  of  Lake  Tre- 
mola.  It  might  be  fancied  that  all  Lombardy  would  be  obscured  by  a 
thick  fog,  but  before  it  has  issued  from  Lake  Tremola,  the  warm  ascend- 
ing currents  dissolve  it. 

Let  us  now  consider  the  clouds  in  themselves,  their  formation,  and 
the  manner  in  which  they  are  suspended  in  space. 

We  saw,  in  the  previous  chapter,  that  the  moisture  of  the  air  increases 
up  to  a  certain  height  until  it  reaches  a  zone  of  maximum  humidity,  the 
elevation  of  which  varies  according  to  the  seasons  and  hours,  and  above 
which  the  air  is  drier  and  drier.  This  zone  was  seen  by  De  Saussure 
in  his  Alpine  travels,  and  by  Commander  Rozet  both  in  the  Alps  and 
the  Pyrenees.  It  is  a  blue,  transparent  vapor,  which  it  is  difficult  to 
distinguish  when  one  is  immersed  in  it,  but  the  upper  surface  of  which 
is  easily  made  out  when  situated  beyond  it.  This  surface  is  always 
horizontal,  like  that  of  the  sea.  From  a  great  height  upon  some  peak 


THE  CLOUDS.  371 

of  the  Alps  or  the  Pyrenees,  the  topmost  limit  of  this  atmosphere  of 
vapor  is  clearly  delineated  on  the  horizon  by  a  bluish  line,  like  that 
which  bounds  the  horizon  of  the  sea.  Its  height  varies  according  to 
the  season  and  hour ;  it  has  been  found  to  vary  between  3500  and  13,000' 
feet.  Its  temperature  never  falls  below  32°. 

It  is  upon  this  surface  of  the  atmosphere  of  vapor  that  clouds  are 
formed,  and  on  which  they  seem  to  repose.  On  the  15th  of  July,  1867, 
I  rose  to  a  height  of  5000  or  6000  feet  before  sunrise,  and  for  once  I 
was  present  at  the  formation  of  clouds  in  the  workshop  of  Nature.  It 
was  above  the  Khine  plain,  between  Cologne  and  Aix-la-Chapelle.  The 
atmosphere  had  remained  pure,  when  small  white  flakes  began  to  ap- 
pear in  the  zone  of  maximum  moisture.  These  gradually  ran  together, 
became  grouped  in  large  numbers,  and  dissolved  with  as  much  rapidity 
as  they  had  formed.  The  small  white  clouds,  agglomerated  together, 
formed  cumuli.  This  formation  of  clouds  was  proceeding  several  hun- 
dred yards  below  us.  As  the  sun  rose,  the  moisture  on  the  balloon 
evaporated,  and  we  gradually  ascended  to  a  height  of  7900  feet.  It 
was  the  same  with  the  clouds,  which  indeed  rose  rather  more  rapidly 
than  the  balloon,  and  finally  surrounded  and  surmounted  it. 

The  clouds  are  generally  carried  along  by  the  wind,  following  its 
course  and  being  relatively  motionless  in  the  current  with  which  they 
float.  The  measurement  of  their  speed  gives,  indeed,  the  measurement 
of  the  velocity  of  the  upper  wind.  But  this  rule  is  not  without  excep- 
tions. There  are,  however,  clouds  which  do  not  progress,  even  when  they 
are  traversed  by  a  more  or  less  powerful  wind,  which  it  would  be 
thought  must  take  them  along  with  it. 

When  traveling  in  company  with  M.  Eugene  Godard  in  a  balloon, 
while  we  were  over  the  forest  of  Villers-Cotterets,  I  was  much  surprised 
to  see  for  more  than  twenty  minutes  a  small  cloud  which  might  have 
been  about  200  yards  in  length  and  150  in  breadth,  suspended  motion- 
less about  eighty  yards  above  the  trees.  As  we  approached,  we  noticed 
five  or  six  smaller,  which  were  disseminated  and  also  motionless,  not- 
withstanding the  air  was  moving  at  the  rate  of  eight  yards  per  second, 
and  we  were  curious  to  ascertain  what  invisible  anchor  retained  these 
small  clouds.  When  we  were  above  them,  we  found  that  the  principal 
was  suspended  over  a  piece  of  water,  and  that  the  others  were  over  the 
course  of  a  stream,  from  which  arose  a  current  of  humid  air,  the  invisible 
moisture  of  which,  reaching  its  saturating  point,  became  visible  in  its 
passage  through  the  cool  wind  that  prevailed  above  the  wood. 


372  THE  ATMOSPHERE. 

Kaemtz  witnessed  an  analogous  occurrence  near  Wiesbaden  after 
heavy  rain.  He  says,  "The  clouds  dividing,  the  sun  burst  forth,  and  I 
saw  a  column  of  rnist  that  continued  to  ascend  from  the  same  point.  I 
hastened  thither,  and  found  a  newly-mown  meadow  surrounded  by  pas- 
ture-lands, the  high  grass  of  which,  being  less  heated  than  the  bare  sur- 
face of  the  mown  meadow,  gave  rise  to  a  less  active  evaporation."  In 
Switzerland  the  phenomenon  occurs  on  a  smaller  scale.  While  it  is 
fine  upon  the  Faulhorn,  the  Swiss  lakes  are  often  covered  with  fogs  of 
very  different  densities.  The  same  meteorologist  has  observed  that  the 
fogs  over  lakes  Zug,  Zurich,  and  Neuchatel  were  very  thick,  while 
those  which  rested  over  lakes  Thun  and  Brienz  were  merely  light 
vapor.  This  phenomenon  has  occurred  too  often  to  be  attributed  to 
chance.  Lake  Zug  is  rather  deep,  and  its  tributaries  do  not  descend 
directly  from  the  regions  of  perpetual  snow.  Its  temperature  must  be 
higher  than  that  of  Lake  Brienz,  into  which  the  Aar  empties  itself  im- 
mediately after  having  descended  from  the  Grimsel  glaciers.  With  the 
temperature  the  same,  the  first  would  become  more  readily  involved  in 
fog  than  the  second. 

I  must  now  explain  the  causes  which  lead  to  the  suspension  of  clouds 
in  the  atmosphere. 

When  a  cloud  is  dissolved  into  rain,  and  pours  down  thousands  of 
gallons  of  water,  the  question  may  well  be  asked  how  it  is  possible  for 
such  a  weight  of  water  to  have  remained  suspended.  The  cause  lies  in 
its  extreme  divisibility.  Left  to  themselves,  the  vesicles  would  fall. 
Calculation  shows  that  it  would  take  them  more  than  half  an  hour  to 
fall  a  little  more  than  one  mile  in  the  atmosphere — that  is  to  say,  that 
the  rapidity  of  their  descent  is  about  one  yard  per  second ;  it  is  often 
less.  But  during  the  day  the  air  is  constantly  traversed  by  warm  as- 
cending currents,  which  rise  with  a  speed  of  several  yards  per  second. 
Thus  the  clouds  can  not  descend  during  day-time  unless  the  circum- 
stances be  exceptional.  It  is  not  necessary  to  suppose  that  their  vesi- 
cles are  filled  with  dilated  and  lighter  air,  as  if  they  were  so  many  small 
balloons.  Nevertheless,  as  Fresnel  has  remarked,  the  solar  heat  ab- 
sorbed by  the  cloud  must  contribute  to  its  remaining  suspended.  At 
night  the  clouds  are  nearer  to  the  ground.  But  we  have  seen  that  the 
conditions,  under  which  the  vapor  of  water  becomes  visible,  depend 
upon  the  temperature  and  the  degree  of  saturation.  It  follows  that  the 
lower  surface  of  the  clouds  dissolves  as  they  descend  into  a  warmer  air, 
and  frequently,  too,  the  upper  surface  dissolves  when  exposed  to  the 


THE  CLOUDS.  373 

action. of  the  sun ;  so  that,  as  a  matter  of  fact,  they  are  constantly  chan- 
ging in  thickness,  shape,  and  .even  substance.  The  clouds  being  but 
water  in  a  special  state,  seem  to  us  motionless  even  when  the  particles 
which  compose  them  are  incessantly  descending  from  their  upper  to 
their  lower  surface,  below  which  they  become  dissolved.  They  rest, 
moreover,  upon  the  zone  of  invisible  vapor  which  I  have  already  spoken 
of.  The  horizontal  march  of  the  currents  represents  a  somewhat  con- 
siderable effort  to  maintain  the  clouds  at  the  same  elevation,  even  when 
all  the  aqueous  particles  are  full. 

Having  dealt  with  the  formation  of  clouds,  and  their  position  in  the 
air,  let  us  consider  their  varied  and  characteristic  shapes. 

The  forms  of  the  clouds  are  of  infinite  diversity,  from  the  thick  fog 
which  bathes  the  surface  of  the  soil  to  the  luminous  detached  filaments 
which  hover  in  the  heights  of  the  atmosphere.  A  methodical  nomen- 
clature of  clouds,  to  enable  observers  to  record  with  precision  observa- 
tions of  their  various  forms,  became  a  necessity.  Howard  first  gave 
names  to  the  principal  types  in  order  to  have  a  means  of  recognizing 
each,  and  his  classification  has  been  generally  adopted,  so  much  so  that 
his  figures  have  become,  so  to  speak,  classic.  His  description  alone  I 
shall  use  as  a  basis  for  my  remarks  on  this  subject. 

In  our  climates  the  clouds  are,  in  most  cases,  rather  oval  in  shape ; 
they  seem  to  be  piled  one  upon  another,  and  their  clearly-defined  edges 
trace  curves  upon  the  azure  of  the  sky.  This  class  of  clouds  have  re- 
ceived the  name  of  cumulus,  and  it  is  in  summer  that  their  shape  is  the 
most  marked.  Sailors  call  them  bales  of  cotton.  They  rise  and  aug- 
ment in  size  during  the  morning;  reach  their  greatest  elevation  when 
the  temperature  is  highest;  from  which  time  they  descend,  and  ulti- 
mately disappear,  when  they  are  not  numerous..  Their  thickness  varies 
from  1300  feet  to  1700  feet;  their. height  from  1500  feet  to  10,000  feet. 

Sometimes  these  half-spheres  become  heaped  one  upon  the  other, 
and  form  those  large,  accumulated  clouds  near  the  horizon  which,  seen 
from  a  distance,  resemble  mountains  covered  with  snow.  These  are 
the  clouds  which  lend  themselves  most  readily  to  the  play  of  the  im- 
agination, for  their  lightness  and  the  extreme  variability  of  their  shape 
give  rise  to  incessant  metamorphoses.  It  is  not  difficult  to  see  in  them 
the  forms  of  men,  animals,  dragons,  trees,  and  mountains.  Ossian  has 
utilized  them  for  some  of  his  finest  imageries.  The  popular  legends 
of  mountainous  regions  are  filled  with  strange  events,  in  which  these 
•  clouds  play  an  important  part. 


374  THE  ATMOSPHERE. 

This  frequently-occurring  shape  is  coincident  with  the  warm  wind 
from  the  S.  and  S.W. — that  is  to  say,  with  the  equatorial  current. 
When  this  moist  current  prevails  for  some  time,  cumuli  become  more 
numerous,  more  dense,  and  spread  in  beds  over  the  sky.  This  second 
form  is  seen  almost  as  often  in  our  variable  climates  as  the  first,  and  it 
is  characteristic  of  winter  as  the  latter  is  of  summer,  the  principal  dif- 
ference being  that  condensation,  or  rain,  takes  place  more  rapidly  when 
the  sky  is  in  this  state  than  it  does  during  the  summer  phase.  This 
kind  of  cloud  is  termed  cumulo-stratus.  The  fleecy  clouds,  the  dappled 
sky,  represent  it  in  well-known  aspects. 

When  the  clouds,  instead  of  being  detached,  form  one  vast  sheet  ex- 
tending to  the  horizon,  the  term  stratus  is  given. 

When  a  cloud  is  about  to  dissolve  in  rain,  it  acquires  a  greater 
density,  becomes  more  sombre,  and,  except  in  the  case  of  hail  or  par- 
tial storms,  extends  over  a  vast  space.  The  water  which  is  discharged 
from  it  would  fall  vertically  if  the  atmosphere  were  calm,  and  the 
drops  of  water  heavy  enough ;  but  two  causes,  of  which  one  at  least  is 
always  in  existence — the  wind,  and  the  lightness  of  the  rain-drops — 
cause  the  water  which  falls  from  the  cloud  to  follow  an  oblique  course, 
generally  preceded  by  the  cloud,  which  the  wind  drives  at  a  greater 
rate  of  speed.  The  special  state  of  the  cloud  resolving  itself  into  rain 
is  termed  nimbus. 

All  these  clouds  are  formed  of  aqueous  vesicles,  more  or  less  consid- 
erable in  size,  and  more  or  less  compact.  But  the  clouds  do  not  only 
reside  in  the  strata,  the  temperature  of  which  is  above  32°;  they  also 
float  in  the  regions  where  the  temperature  is  below  the  freezing-point. 
In  this  state  the  vesicular  water  becomes  congealed  into  minute  fila- 
ments of  ice,  and  the  clouds  formed  in  this  way  are  clouds  of  ice  or 
snow,  which  have  already  served  to  explain  such  optical  phenomena 
as  halos,  parhelia,  etc.  These  clouds  of  ice  are  those  which  reach  the 
loftiest  regions.  No  matter  the  height  to  which  the  balloon  may  rise, 
these  clouds  always  appear  so  far  above  that  they  seem  no  nearer  than 
when  viewed  from  the  earth ;  whereas  it  is  a  work  of  scarcely  any  time 
to  travel  through  cumuli  and  the  other  forms  of  clouds  which  I  have 
mentioned.  Mr.  Glaisher  found  that  at  37,000  feet  above  the  soil  of 
England,  he  was  still  far  below  them. 

They  are  composed  of  loose  filaments,  the  ensemble  of  which  is 
sometimes  like  the  sweep  of  a  broom,  sometimes  like  a  bunch  of  feath- 
ers, sometimes  like  a  mass  of  hair,  or  a  ligl  t  and  irregular  piece  of  net- 


THE  CLOUDS.  375 

work.  Their  mean  height  is  from  twenty  to  twenty-three  thousand 
feet. 

By  reason  of  their  very  constitution,  they  remain  in  the  ethereal 
regions  of  eternal  snow.  But,  as  I  have  said,  the  zone  of  32°  varies  in 
height  according  to  climates  and  season,  whence  it  follows  that  these 
clouds  may  make  their  appearance  in  the  lower  regions  of  the  atmos- 
phere in  the  frosty  latitudes  of  the  polar  regions,  and  even  in  our  lati- 
tudes during  a  severe  frost. 

These  clouds  are  designated  cirrus.  With  a  little  practice  it  is  easy 
to  recognize  them,  and  what  is  most  striking  in  them  is  that  they  are 
nearly  always  divided  into  long  and  narrow  strips,  quite  straight,  and 
white  in  color,  which  correspond  with  the  upper  currents  that  direct, 
mold,  or  dissolve  them. 

Sometimes  their  whitish  hue  gets  bedimmed.  their  striae,  interlace 
each  other,  and  they  become  denser  because  the  upper  air  is  moist.  In 
this  case  they  look  like  carded  cotton,  and  this  change  generally  fore- 
tells rain.  When  in  this  state  of  excessive  density,  they  are  called 
cirro-stratus. 

Sometimes,  too,  they  become  transformed  into  light  transparent 
clouds  of  vesicular  vapor — so  transparent  that  the  stars  and  the  spots 
on  the  moon  can  be  seen  through  them.  These  are  clouds  which  give 
rise  to  the  coronce;  when  they  are  in  receipt  of  abundant  light,  they 
seem  to  be  well  rounded  and  fleecy;  when  the  sky  is  covered  with 
them,  it  is  said  to  be  dappled ;  their  mean  height  is  from  ten  to  thirteen 
thousand  feet;  they  are  termed  cirro-cumulus.  The  cumulus  and  the 
cirro-cumulus  are  those  which  impart  the  most  beautiful  hues  to  sun- 
set; their  transparency  and  their  distant  reflection  refracting  and  color- 
ing its  rays.  The  beautiful  sunsets  seen  in  Paris  are  partially  due  to 
the  fact  that  these  clouds,  situated  above  Havre  for  the  horizon  of 
Paris,  give  us  a  softened  reflection  of  the  luminous  effects  that  are  pro- 
duced by  the  sea. 

Such  are  the  principal  shapes  which  clouds  take,  and  which  are  due 
to  the  difference  in  their  constitution  and  their  elevation.  These  va- 
rieties do  not  constitute,  in  reality,  more  than  two  great  categories — 
the  cumulus,  formed  of  liquid  vesicles,  and  the  cirrus,  formed  of  frozen 
particles. 

M.  A.  Poey  gives  the  following  "  scientific  and  popular  classification" 
of  the  various  shapes  of  clouds : 


376  THE  ATMOSPHERE. 

1st  Type.-CiRBirs.    Curly  cloud J  Prozen  douds     Height)  26)000  to  ^m  feet. 

f  Cirro-stratus.    Streaky  cloud > 

Derivatives.  ]  Cirro-cumulus.    Dappled  cloud J  gnow  dond8     Hejght)  ^m  ^  ^m  feet 

{Pallio-cirrus.    Cloud  in  strata ' 

2d  Type.— CUMULUS.    Mountainous  cloud ") 

{Pallio-cumulus.    Rain  cloud Ram  douds'  veslcnlar  Or  °f  vaPor  of  watei'' 


terivatives.  ^^^^^    wind  cloud )     Average  height,  3200  feet. 

Among  the  clouds  composed  of  liquid  vesicles,  we  must  now  con- 
sider the  peculiar  and  characteristic  shapes  corresponding  to  the  pro- 
duction of  aqueous  meteors,  of  which  they  are  either  the  cause  or  the 
forerunner. 

My  colleague,  3.  Silbermann,  Vice-president  of  the  Meteorological 
Society,  has  spent  thirty  years  in  studying  and  making  designs  of  these 
specially  typical  shapes.  Out  of  the  large  number  which  he  has  stereo- 
typed and  collected  in  a  kind  of  meteorological  museum,  I  will  cite  the 
principal. 

Every  one  is  acquainted  with  the  shape  of  the  clouds  which  usher  in 
a  lengthy  period  of  rain ;  the  sky  is  covered  with  an  immense  leaden 
sheet,  and  the  rain  falls  continuously  from  horizontal  strata  slightly 
undulated,  which  are  scarcely  distinguishable  from  the  sombre  mass  in 
its  entirety.  For  days  and  nights  together  the  sky  continues  covered 
with  this  opaque  sheet,  the  thickness  of  which  is  sometimes  many  thou- 
sand yards,  there  being  successive  strata  by  which  the  light  of  the 
autumn  sun  is  entirely  absorbed.  These  are  clouds  of  continental  rain, 
which  extend  over,  vast  tracts  of  country,  and  the  contour  of  which  it 
is  impossible  to  make  out. 

The  clouds  of  partial  rain  resemble  them  so  far  that  they  are  length- 
ened into  horizontal  strata;  but  in  this  case  their  shape,  less  extended, 
is  more  definite,  as  it  stands  out  against  the  background  of  the  sky, 
which  is  no  longer  darkened  by  the  immensity  of  the  strata  that  lie  one 
over  the  other,  but  is  partially  covered  with  cumuli  that  have  different 
densities  in  different  places.  The  rain  issues  from  the  sides  of  the 
clouds;  it  is  delineated  upon  the  pale  perspective  of  the  sky  in  oblique 
streaks  of  gray,  the  general  tone  of  which  varies  with  the  motion  of 
the  wind.  These  clouds  do  not  always  dissolve  entirely;  certain  parts 
seem,  after  they  have  discharged  a  great  quantity  of  rain,  to  dry  up 
and  fall  back  into  the  centre  of  the  cloud,  as  if  attracted  by  the  molec- 
ular affinity  which  gives  to  clouds  their  varying  contour. 

The  hail-squall  is  different;  it  does  not  spread  out  in  a  large  hori- 
zontal sheet,  but  forms  a  definite  mass,  which  often  stands  out  by  itself 


THE  CLOUDS. 


377 


in  the  blue  sky.  The  sun  reaches  to  its  edges  and  sets  off  its  white  sur- 
face against  the  rest  of  the  sky;  there  issues  from  its  open  sides  a  cold 
rain,  hail,  and  rime,  which  a  March  wind  blows  into  our  faces. 

The  clouds  which  produce  Whave  the  singular  aspect  of  an  adhe- 
sion of  moleculae,  as  if  attraction  tended  to  unite  them  in  condensed 
masses  of  a  globular  form,  and  their  shape  has  a  strange  resemblance  to 
that  of  a  cauliflower.  This  peculiar  adhesion  has  also  been  noticed  in 
thunder-clouds;  the  lower  plane  of  this  species  of  cloud  is  horizontal, 
and  from  .this  kind  of  table-like  base  rise  projections,  the  shape  of  which 


Fig.  68. — Formation  of  a  thunder-cloud. 

may  be  compared  with  enormous  balls  of  wool  more  or  less  carded,  and 
connected  the  one  with  the  other.  These  are  typical  instances  which 
accentuate  rather  than  attenuate  the  average  appearance  of  clouds.  The 
color,  the  white  or  the  sombre  hue  of  the  clouds,  can  scarcely  be  taken 
as  characteristic,  for  they  are  dependent  upon  their  position  in  respect 
to  the  sun,  and  in  regard  to  the  situation  of  the  observer. 

If  we  see  a  cloud  at  a  great  distance,  and  are  standing  between  it  and 
the  sun,  it  will  seem  to  us  to  be  white.     If,  on  the  contrary,  we  notice  it 


378  THE  ATMOSPHERE. 

as  it  passes  over  our  heads,  we  see  the  lower  surface  which  the  light 
does  not  reach,  and  then  it  appears  black. 

The  snow  clouds  have  not  this  definite  shape.  They  generally  are  of 
an  immense  thickness  in  the  atmosphere,  and  of  slight  density.  The 
light  sifted  athwart  their  vast  extent  gives  them  a  yellowish  tint,  whence 
the  flakes  descend  and  cover  the  earth. 


Fig.  69.— Above  and  below  the  rain  clond. 


JtALV. 


CHAPTER  III. 

RAIN :    GENERAL  CONDITIONS  OF  THE   FORMATION  OF  RAIN ITS  DIS- 
TRIBUTION  OVER  THE   GLOBE — RAIN  IN  EUROPE. 

HAVING  treated  of  the  distribution  of  moisture  in  the  atmosphere,  the 
manner  in  which  the  clouds  are  formed  and  remain  suspended  in  space, 
their  division  into  two  distinct  kinds,  and  the  action  of  temperature 
upon  the  vapor  of  water,  we  shall  have  no  difficulty  in  discovering  how 
the  formation  of  rain  takes  place. 

Ruin  is  the  precipitation  of  the  aqueous  vapor  which  constitutes  the 
clouds.  For  this  vapor  to  become  precipitate — that  is,  to  form  drops, 
the  weight  of  which  causes  them  to  descend  and  to  produce  rain — the 
molecular  state  of  the  cloud  must  be  modified  by  some  external  cause. 
This  modification  may  be  effected  by  the  influence  of  upper  clouds — 
clouds  of  ice.  Under  certain  circumstances,  the  least  decline  of  tem- 
perature sets  them  in  motion  and  destroys  them.  Such  is  the  case  with 
saturated  cumuli;  the  least  diminution  of  temperature  precipitates  them 
in  the  form  of  rain. 

The  ordinary  condition  of  the  production  of  rain  consists,  therefore, 
in  the  existence  of  two  layers  of  clouds,  one  above  the  other,  and  it  is 
the  higher  which  causes  the  precipitation  of  the  one  below  it.  This  is 
an  observation  which  any  one  may  verify  for  himself;  and  in  the  course 
of  many  years'  observation  of  the  sky  when  rain  is  about  to  fall,  I  have 
never  found  this  condition  wanting. 

Monck  Mason  remarked,  in  his  aeronautical  voyages,  that  when  rain 
falls,  the  sky  being  at  the  time  totally  covered  with  clouds,  there  is  al- 
ways a  similar  range  of  clouds  situated  at  a  certain  height  above,  and 
that  when,  on  the  contrary,  though  it  does  not  rain,  the  sky  presents 
the  same  appearance  below,  bright  sunshine  prevails  in  the  space  im- 
mediately above.  Saussure  had  already  noted  the  same  fact  in  his  Al- 
pine explorations.  Hatton  had  noticed  that  when  two  masses  of  air, 
saturated  or  nearly  saturated,  but  of  unequal  temperature,  meet,  there  is 
a  precipitation  of  aqueous  vapor.  Peltier  observed  in  regard  to  another 
point,  that  a  thunder-storm  is  always  composed  of  two  banks  of  clouds 
which  are  of  opposite  electricity.  Rozet  arrived  at  the  conclusion  that 


382  THE  ATMOSPHERE. 

thunder-storms  and  rain  both  result  from  the  encounter  between  the 
cirrus  and  the  cumulus,  between  the  frozen  and  the  vesicular  vapor. 
Kaemtz  and  Martins  adopt  the  same  theory.  M.  Eenou  further  adds 
that  water  may  fall  withput  being  frozen  at  temperatures  as  low  as  27°, 
36°,  or  45°  below  the  freezing-point  of  water,  in  the  state  of  extreme  di- 
visibility which  constitutes  fogs  and  mists,  and  that  rain  and  frost  are 
due  to  the  admixture  of  frozen  cirrus  with  the  still  liquid  cumulus  be- 
neath the  varying  influence  of  temperature. 

Such  is  the  general  manner  in  which  rain  is  formed.  It  sometimes, 
however,  falls  when  the  sky  is  clear.  On  August  9, 1837,  at  9  P.M., 
Wartmann  of  Geneva  noticed  that  during  the  space  of  two  minutes 
large  drops  of  warm  rain  fell  from  the  sky,  then  studded  with  stars. 
The  edges  of  the  horizon  were  covered  with  broken  patches  of  black 
clouds. 

On  the  31st  of  May,  1838,  at  7  P.M.,  M.  Wartmann  again  remarked  an 
analogous  phenomenon,  which  this  time  lasted  for  six  minutes.  The 
warm  drops,  which  were  at  first  very  large  and  thick,  gradually  de- 
creased in  size.  On  the  llth  of  May,  1844,  at  10  A.M.  and  3  P.M.,  he  no- 
ticed the  same  occurrence,  and  during  the  time  the  air  being  quite 
calm. 

The  transit  of  masses  of  clouds  is  an  important  factor  in  their  disso- 
lution, and  in  the  abundance  and  the  distribution  of  rain.  This  has 
been  already  pointed  out  when  we  were  considering  how  the  various 
directions  of  the  wind  corresponded  with  the  amount  of  rain  that  fell. 
The  south-west  wind,  which  prevails  in  our  country,  brings  the  greatest 
amount  of  rain,  because  it  is  accompanied  by  the  cloudy  strata  formed 
over  the  ocean,  these  strata  of  humidity  being,  moreover,  sometimes  in- 
visible. 

Thus  we  can  form  an  idea  of  the  immense  evaporation  which  daily 
takes  place  from  the  surface  of  the  ocean,  and  see  in  it  the  origin  of 
clouds  and  rain.  The  trade- winds,  which  blow  over  the  surface  of  the 
sea  in  the  tropics,  carry  this  vapor  of  water  as  far  as  the  regions  of  equa- 
torial calm,  where  they  rise  into  the  higher  a"nd  colder  part  of  the  at- 
mosphere, and  from  thence  pass  to  the  temperate  countries  laden  with 
moisture.  As  they  rise  through  the  atmosphere  in  the  equatorial  re- 
gions, a  portion  of  vapor  is  condensed ;  and  as  this  occurs  every  day, 
there  is  a  constant  zone  of  clouds  and  rain.  It  is  what  English  sailors 
term  the  cloud-ring,  and  French  sailors  the  Pot  au  Noir. 

The  oceanic  clouds  from  the  south  and  the  south-west  distribute  the 


SAIN.  M6 

water  which  they  contain  according  to  their  course,  height,  and  temper- 
ature ;  the  more  or  less  thick,  and  more  or  less  cold,  strata  of  clouds 
which  weigh  down  upon  them  varying  with  the  accidental  winds  which 
may  affect  them,  and  influenced  by  the  undulations  of  the  ground  which 
alter  their  course.  All  other  conditions  being  unchanged,  the  propor- 
tion of  rain  decreases  from  the  equator  to  the  poles,  since,  on  the  one 
hand,  evaporation  takes  place  almost  entirely  in  the  warm  latitudes; 
and,  on  the  other,  the  quantity  of  vapor  which  the  air  is  capable  of  dis- 
solving augments  rapidly  as  the  temperature  increases.  Thus,  for  in- 
stance, there  is  an  annual  rain-fall  of  more  than  six  and  a  half  feet  at 
Guiana  and  Panama,  while  it  is  only  seven  and  three-quarter  inches  at 
Archangel. 

United  Sutes  Tropical  Zone  S oath  of  tlte  Alps    North  at  the  Ops  • 

ScmdiaA 


Fig.  70.— Diminution  in  the  raiu-fall  from  the  tropics  to  the  poles. 

There  is  also  a  second  law  in  regard  to  the  proportion  of  rain,  viz., 
that  it  diminishes  in  amount  according  to  the  distance  from  the  sea, 
measured  in  the  direction  of  the  prevailing  winds.  It  is  easy  to  under- 
stand that  clouds,  being  unable  to  reform  in  the  interior  of  continents, 
yield  less  rain  in  proportion  as  they  pass  farther  from  the  ocean.  The 
evaporation  that  proceeds  from  rivers,  lakes,  pools,  and  moist  plains, 
does  indeed  give  rise  to  clouds ;  but  this  is  a  very  insignificant  source 
of  rain  compared  to  that  of  the  ocean.  There  falls  forty-nine  inches 
nearly  at  Bayonne  ;  forty-seven  inches  at  Gibraltar;  fifty-one  inches  at 
Nantes;  only  sixteen  and  a  half  inches  at  Frankfort;  seventeen  and 
three-quarter  inches  at  St.  Petersburg  and  Vienna.  In  Siberia  the  rain- 
fall is  but  seven  and  three-quarter  inches,  and  less  still  farther  east. 
At  Algiers  there  is  a  mean  of  seven  and  three-quarter  inches,  and  at 
Oran  and  Mostaganem  of  less  than  four  inches.  Farther  south,  the 
quantity  of  rain  diminishes  rapidly ;  and  at  Biskra,  on  the  borders  of 
the  desert,  there  falls  two-tenths  of  an  inch  in  the  course  of  the  year. 

Numerous  observations  enable  us  to  establish  a  third  law.  The  un- 
dulating nature  of  the  ground  causes  a  variation  in  the  two  distributing 
elements  which  we  have  just  been  considering.  If  a  mass  of  air,  satu- 
rated with  moisture,  encounters  a  mountain  chain,  it  will  be  partially 
stopped  by  this  protuberance  of  the  soil.  But  the  check  is  not  a  long 


384  THE  ATMOSPHERE. 

one.  The  currents  of  air  which  ascend  the  slopes  of  mountains  will 
elevate  them  at  the  same  time  ;  they  will  become  colder  at  the  rate  of 
one  degree  to  200,  250,  or  330  feet  ;  according  to  the  season  and  tem- 
perature, they  will  consequently  undergo  a  progressive  condensation,  so 
that  when  they  reach  the  summit  they  will  be  able  to  pass  above  it;  a 
great  part  of  the  water  they  contained  will  already  have  fallen,  and  the 
remainder  will  descend  upon  the  summit  of  the  mountain.  The  lessen- 
ed speed  of  the  air  also  deprives  them  of  their  water,  much  in  the  same 
way  as  the  diminishing  rapidity  of  a  stream  facilitates  the  fall  of  the 
deposits  which  it  keeps  suspended.  There  falls,  therefore,  more  rain  in 
a  mountainous  than  in  a  level  region  ;  there  is  also  more  rain  upon  the 
slope  that  faces  the  sea-wind  than  upon  the  opposite.  Thus,  clouds 
which,  as  they  pass  over  Lisbon,  give  but  an  annual  rain-fall  of  twenty  - 


Altitudes    2600 


Fig.  71.— Increase  of  rain,  according  to  the  undulations  of  the  soil 

seven  and  a  half  inches,  are  soon  arrested  by  the  cold-tipped  mount- 
ains of  Portugal  and  Spain,  there  being  a  rain-fall  of  118  inches  at  Coim- 
bra.  The  clouds  which  pass  at  the  zenith  of  Paris  yield  nineteen  and 
three-quarter  inches  of  rain  in  a  year.  As  the  altitude  augments,  so 
does  the  rain.  Thus,  taking  merely  the  basin  of  the  Seine,  we  have 
three  and  a  quarter  feet  of  rain-water  upon  the  plateau  of  Langres, 
and  six  feet  nearly  at  the  higher  point  of  Morvan,  in  the  Nievre.  At 
Geneva,  at  the  foot  of  the  Alps,  the  annual  quantity  of  rain  is  thirty- 
two  and  a  half  inches,  and  at  the  Great  St.  Bernard  ridge  it  is  six  and 
a  half  feet  in  the  year. 

There  are  regions  in  which  these  conditions  are  so  complete,  that  the 
rain  stops  as  if  attracted  there  permanently.  Thus  the  Great  Himalaya 
chain  stops  the  clouds  which  come  from  the  immense  evaporation  of 
the  Indian  Ocean.  At  Cherra-Poejen,  upon  the  Garrows  Mountains,  at 
a  height  of  4500  feet,  and  to  the  south  of  the  Brahmapootra  Valley, 
the  quantity  of  rain  which  the  clouds  pour  down  is  forty-eight  and  a 
half  feet.  In  these  mountainous  regions  near  the  tropics,  the  maximum 
rain-fall  is  probably  to  be  found ;  they  are  also  the  great  reservoirs  of 
the  Asiatic  rivers.  In  these  same  lower  slopes  of  the  Himalayas,  upon 


JIADf. 

385 


he  eastern  s,de  of  the  Ghauts,  an  average  annual  rain.fall  of  twenty 
five  feet  nearly  has  been  recorded,  after  observations  extending  ov" 
penod  of  fourteen  years.     A  downfa,,  lasting  only  foar  houra  Lbeen 
known  to  cover  the  ground  to  a  depth  of  thirty  inches-more  than  Ms 
at  Pans  m  a  whole  yea,     It  is  cemdn  that  in  no  other  part  of  I  to 
nd  zone  ,s  the  precipitation  of  the  rain  so  much  facilitated  by  attendant 
cn-cumstances.     The  Antilles  are  not  wide  enough  to  prevent  the  ± 
and  clouds  from  veenng  obliquely  to  the  right  or  to  the  left;  but  not- 
w,thstand,ng,  certain  districts  there  receive  tbirty-.wo  and  three-quar- 
ter  feet  m  the  course  of  the  twelvemonth.    In  the  Gulf  of  Mexico  the 
summer  rains  also  give  a  depth  of  more  than  thirteen  feet  at  Ten 
Cruz.     Farther  from  the  tropical  regions  we  only  noticed  these  remark 
le  max,™  of  ram  upon  the  mountain  chains  which,  being  in  the  wav 


o  : 


Cherra-Poejen.«  Mahabnleshwar.,   Vera  Cruz.     Bergen.         Name*. 
Fig.  72.— Comparative  depths  of  rain-fall. 

of  the  general  current,  bring  it  to  a  stop.  Such,  for  instance,  is  the  ef- 
fect produced  by  the  Scandinavian  Alps  that  separate  Sweden  and 
Norway,  for  its  western  slope  receives  much  more  rain  than  the  eastern 
side,  there  being  an  annual  rain-fall  of  eight  and  three-quarter  feet  at 
Bergen,  which  exceeds  that  of  any  other  town  in  Europe.  Moreover, 
several  points  are  again  specially  favored  in  respect  to  their  frontage  to 
the  south-west  current ;  as  Nantes,  for  instance,  where  there  is  a  mean 
annual  rain-fall  of  four  and  a  quarter  feet. 

Collecting  and  comparing  the  observations  that  have  been  made  at  a 
great  number  of  places  in  different  parts  of  the  globe,  it  has  been  found 
possible  to  register  the  three  predominating  causes  which  we  have  re- 

*  [At  Cherra-Poejen  the  fall  of  rain  in  April  is  22  inches  ;  in  May,  62  inches ;  in  June, 
195  inches;  in  July,  121  inches;  in  August,  104  inches;  in  September,  75  inches;  and  in 
October,  29  inches  ;  making  a  total  fall  in  seven  months  of  608  inches.  No  rain  falls  either 
in  November  or  December,  and  less  than  five  inches  in  the  months  of  January,  February, 
and  March.  See  my  "Report  on  the  Meteorology  of  India,"  in  relation  to  the  health  of  the 
troops  stationed  there,  1863.— ED.] 

25 


386  THE  ATMOSPHERE. 

viewed,  to  lay  down  upon  a  diagram  the  depths  of  the  rain-fall  that 
have  been  observed,  and  to  make  a  map  exhibiting  the  comparative 
depth  of  rain  all  over  the  globe.  The  heaviest  rain  takes  place  to  the 
north  of  the  equator  in  the  Atlantic,  in  the  Pacific,  and  to  the  east 
of  America.  In  these  regions,  the  maximum  falls  exceed  six  and  a 
half  feet  in  depth ;  in  Asia,  in  the  islands  of  Borneo,  Sumatra,  and 
Java,  along  the  Himalaya  and  Ghauts  Mountains;  in  Africa,  along  the 
table-lands  of  the  eastern  coast;  in  the  Atlantic,  between  Guinea  and 
Guiana ;  in  South  America,  upon  the  Andes  in  Chili,  at  Cape  Horn, 
and  upon  the  summit,  above  Peru,  which,  by  contrast,  is  a  country 
where  no  rain  falls.  Lastly,  the  mountain  chain  which  runs  eastward 
along  the  borders  of  North  America,  from  fifty  to  sixty  degrees  longi- 
tude, yields  an  annual  maximum  of  more  than  six  and  a  half  feet. 

The  rainless  regions  extend  along  the  desert  of  Sahara,  Egypt,  Ara- 
bia, and  Persia,  reaching  as  far  as  Mongolia,  and  even  to  Siberia,  with 
the  exception  of  the  region  of  Central  Asia,  upon  which  the  monsoons 
and  the  winter  rains  yield  some  little  moisture. 

If  we  consider  Europe  in  particular,  we  find,  relatively,  abundant 
rain,  ranging  from  three  and  a  quaiter  to  six  and  a  half  feet,  in  the 
marine  zones  of  Portugal,  Brittany,  Ireland,  and  Sweden.  The  propor- 
tion of  rain  gradually  diminishes  toward  the  east,  with  the  zones  of 
condensation  produced  by  the  undulating  nature  of  the  soil.  There 
are  certain  points  where  rain  is  very  rare,  as  in  Greece,  for  instance. 
The  climate  of  Attica  is  dry,  and  the  sky  is  generally  clear,  the  air 
having  always  been  considered  the  purest  in  Greece.  As  an  instance 
of  this,  I  may  mention  that  M.  Lusieri  exposed  a  piece  of  paper  to  the 
air  all  night,  and  that  he  was  able  to  write  upon  it  the  next  morning. 
To  this  remarkable  dryness  of  the  air  has  been  attributed  the  excellent 
state  of  preservation  of  the  Athenian  monuments. 

The  northern  hemisphere  receives  more  rain  than  the  southern  by 
about  one-fourth.  This  excess  of  rain  is  especially  due  to  the  northern 
equatorial  zone  of  rains  and  monsoons.  Nevertheless,  there  is  much 
more  dry  land  in  the  former  than  in  the  latter,  and  evaporation  pro- 
ceeds on  a  much  larger  scale  in  the  southern  hemisphere,  which  is  near- 
ly all  sea.  Thus,  our  clouds,  our  rain,  our  rivers,  and  our  streams  are 
chiefly  fed  by  the  ocean  in  the  hemisphere  of  our  antipodes. 

As  the  distribution  of  rain  has  for  its  origin  both  the  variations  of 
temperature  and  the  prevailing  winds,  it  can  be  easily  seen  that  in  dif- 
ferent countries  it  is  more  or  less  abundant  according  to  the  time  of  year. 


387 

The  countries  in  which  there  is  what  is  termed  a  rainy  season  are 
those  situated  in  the  tropics,  where  the  sun,  which  twice  a  year  passes 
perpendicularly  over  them,  occasions  at  those  epochs  an  excessive  heat, 
which  must,  of  course,  be  succeeded  both  by  a  great  rarefaction  of  the 
strata  next  to  the  ground ;  as  these  latter,  becoming  too  light  to  bear 
the  weight  of  the  upper  strata,  rise,  and  afterward  by  the  diminution 
of  temperature  and  fall  of  rain,  which  always  follow,  no  matter  what 
may  have  been  the  producing  cause.  It  is  impossible  to  form  an  idea 
of  the  mass  of  water  which,  during  the  rainy  season,  falls  into  the 
basins  of  the  Amazon  and  the  Orinoco.  After  these  streams  and  their 
tributaries  have  overflowed  their  banks  to  a  height  of  several  feet,  a 
tract  of  country  as  large  as  Europe  becomes  a  fresh-water  sea,  the  out- 
flow of  which  into  the  ocean  destroys  the  salt  for  some  distance  from 
the  shore,  and  in  comparison  with  which  the  North  American  lakes  are 
mere  mill-ponds.  The  scientific  study  of  this  great  display  of  physical 
forces,  in  which  Nature,  whose  action  is  irresistible,  commands  the  at- 
tention of  us  whose  existence  is  menaced,  is  making  rapid  progress,  and 
none  are  better  qualified  to  throw  light  upon  the  subject  than  the  in- 
habitants themselves,  whose  life  depends  upon  their  being  familiar  with 
the  vicissitudes  of  the  seasons. 

Thus,  in  the  United  States,  upon  the  Atlantic,  from  the  twenty- 
fourth  and  as  far  as  the  fortieth  degree  of  latitude,  in  Spain,  in  the 
south  of  France,  in  Italy,  Greece,  Turkey,  Asia,  China,  Japan,  and  in 
the  Pacific,  in  the  same  latitudes,  nearly  all  the  rain  falls  in  winter,  ex- 
cepting the  region  of  periodical  monsoons  and  in  certain  southern  coun- 
tries, where,  during  the  summer  months,  no  cloud  appears  in  the  sky. 
It  is  the  same  between  the  twenty-fifth  and  fortieth  degrees  of  south 
latitude,  at  Buenos  Ayres,  the  Cape,  and  at  Melbourne. 

Over  a  zone  extending  from  twelve  to  fifteen  degrees  of  south  lati- 
tude, over  nearly  all  the  globe,  it  is  in  summer  that 'most  rain  falls. 

Over  a  zone  extending  from  forty  to  sixty  degrees  of  north  latitude, 
and  which  reaches  as  far  as  seventy-five  degrees,  beyond  Iceland  and 
Sweden,  and  within  a  limited  zone  in  Asia,  rain  falls  at  all  times  of  the 
year. 

Nevertheless,  even  in  our  variable  regions,  there  are  well-defined 
proportions  for  each  particular  season.  Thus,  taking  France  in  partic- 
ular, we  find  that  it  may  be  divided  into  two  parts.  The  western  re- 
gion has  the  maximum  of  rain  in  summer,  and  the  minimum  in  winter. 
Such  is  also  the  case  in  England,  while  in  Germany  it  is  the  reverse, 


388 


THE  ATMOSPHERE. 


under  even  more  marked  conditions.  The  same  holds  good  with  re- 
gard to  Eussia. 

We  have  said  that  there  is  an  annual  rain-fall  of  seven  and  a  quarter 
feet  at  Bergen,  in  Norway.  This  town  is,  in  this  respect,  a  remarkable 
exception  in  the  meteorology  of  the  globe.  It  is,  in  all  Europe,  the 
town  where  there  is  the  most  rain.  It  is  situated  in  the  centre  of  a 
deep  bay,  exposed  to  westerly  winds,  which  are  stopped  by  the  mount- 
ains, so  that  the  rain  is,  to  use  Kaemtz's  expression,  mechanically 
pressed  out. 

The  following  table  gives  the  rain-falls  throughout  Europe,  and  is 
the  result  of  many  years'  observations : 

QUANTITY  OF  RAIN  IN  EUROPE  BY  SEASONS. 


Names  of  Places. 

1 

i 

to 

° 

s. 

GO 

I 

1 

Autumn. 

1 

Number  of 
Observations. 

f 

| 

& 

Breslau  . 

Iu. 
2-2 

In. 
3'0 

In. 
5  '5 

In. 
3'2 

In. 
13-9 

1  * 

Feet. 
460 

51  6 

Prague    

2'2 

3'7 

6'3 

3-1 

15  '3 

~>2 

627 

50  5 

Upsal  

2'7 

2-9 

5'6 

4-5 

15'7 

102 

59  52 

Vienna  

3'3 

3'9 

6'5 

4'0 

17-6 

15 

512 

48  13 

St  Petersburg 

2  '9 

2'9 

6'7 

5'1 

17'6 

16 

59  56 

5-4 

5-5 

6  '9 

7-5 

25  "2 

55 

51  31 

Berlin 

4-4 

4-3 

7'1 

4'6 

20  '4 

12 

128 

52  34 

Paris. 

4-1 

4-6 

5-4 

5-6 

19'7 

140 

285 

48  50 

Stockholm  

3-0 

3-3 

7  "6 

6'7 

20*6 

36 

135 

59  21 

8  '4 

5  -2 

1*3 

8*0 

22  '8 

24 

38  8 

Copenhagen  

5-0 

4'6 

7'1 

6'3 

23-0 

42 

55  41 

Abo 

4'7 

3-9 

7  -2 

7-9 

23  -7 

48 

60  27 

Stuttgart. 

4-2 

5-7 

8*5 

5-9 

24  -3 

31 

814 

48  46 

Toulouse    . 

5-2 

7'0 

5-9 

6  '6 

24  -7 

25 

499 

43  36 

Metz  

5-6 

5-7 

7-2 

7-4 

25  -9 

22 

49  7 

5-7 

6'1 

7'0 

8*5 

27  '3 

30 

47  19 

5  '8 

5'0 

6'7 

7  '4 

24  '9 

27 

289 

55  57 

Brussels  . 

6'4 

6  '2 

8  '3 

7'6 

28  '5 

21 

50  51 

Rouen  

7-6 

6*8 

7'1 

3-9 

30  -4 

26 

190 

49  26 

Ghent  

6'5 

6'5 

9-5 

8*4 

30  '9 

16 

36 

51  3 

Dublin  

6-8 

5-9 

8-1 

8  '4 

29-2 

16 

53  23 

Rome 

9-3 

7-3 

3-4 

10  '9 

31  '0 

40 

174 

51  54 

Geneva 

5  '2 

7  "2 

9-0 

ll'O 

32  -4 

29 

1299 

46  I9 

Montpellier 

9  '2 

7-2 

4'1 

11  '9 

32  -4 

26 

43  36 

Padua        

7'0 

7-4 

9'0 

10  "6 

34  '0 

48 

45  94 

Manchester  

8'2 

7'0 

9-9 

10'7 

35  '7 

47 

154 

53  29 

Florence  

10-2 

8'6 

5-3 

12  '7 

36  -8 

16 

210 

43  47 

Turin 

5  '6 

11  '3 

11*2 

9*5 

37'6 

15 

915 

45  4 

Milan 

8'1 

9'1 

9  '2 

11'7 

38  '1 

68 

479 

45  28 

6'1 

8-1 

14  '9 

11  '2 

40  -3 

6 

1663 

46  31 

Nicolaief.  

14-5 

9-1 

24-8 

14-6 

63-0 

6 

465 

The  quantity  of  rain  at  Breslau,  Prague,  Upsal,  Vienna,  and  St. 
Petersburg,  shows  'how  little  falls  in  these  places,  as  the  mean  is  less 
than  15|  inches. 


BAIN.  339 

In  the  Netherlands,  Belgium,  France,  German}',  and  Poland,  the 
average  is  19£,  23£,  and  27-|  inches.  It  is  easy  to  see  that  there  is  a 
diminution  as  one  recedes  from  the  sea  inland.  Thus,  in  the  Belgian 
cities,  there  is  more  than  27£  inches  of  rain,  while  in  the  same  latitudes 
at  the  German  towns  and  those  nearer  to  Asia  the  quantity  is  much 
smaller.  Upon  the  other  hand,  it  is  evident  that  the  two  most  rainy 
seasons  are  summer  and  autumn,  no"  matter  what  the  distance  of  the 
locality  from  the  sea.  England  is  very  peculiarly  situated  in  this  re- 
spect, as,  being  surrounded  by  the  sea,  she  receives  more  rain  than  her 
latitude  would  lead  one  a  priori  to  expect. 


390  THE  ATMOSPHERE. 


CHAPTER  IV. 

HAIL  :  PRODUCTION  OF  HAIL  —  COURSE  OF  HAILSTORMS  —  VARYING 
DISTRIBUTION  OF  HAILSTORMS  IN  DIFFERENT  PARTS  OF  THE  COUN- 
TRY—  HEAVIEST  HAILSTORMS  KNOWN — NATURE,  SIZE,  AND  SHAPE 
OF  HAILSTONES — PERIODS  OF  THEIR  OCCURRENCE. 

WHEN  several  strata  of  black  and  grayish  clouds  are  flying  through 
the  atmosphere,  and  when  the  thunder-storm  has  burst  forth,  millions 
of  pounds  of  hailstones  are  launched  from  the  clouds  as  if  precipitated 
from  the  opened  cataracts  of  a  vast  reservoir.  For  several  minutes  the 
hail  drives  through  space,  pelting  trees  and  gardens ;  it  then  ceases  as 
the  wind  blows  it  off  in  some  other  direction,  and  the  close  and  sultry 
temperature  which  had  preceded  it  gives  place  to  the  fresh  odor  of 
refreshed  plants,  light  returns,  the  rainbow  appears,  and  the  blue  sky 
emerges  from  the  banks  of  clouds.  What  is  the  force  which  produces 
in  the  clouds  these  lumps  of  ice  (often  very  large),  what  bears  them  up 
in  space,  and  then  launches  them  upon  the  earth  ?  While  studying  the 
production  of  rain,  we  saw  that  it  does  not,  as  a  rule,  occur  except  when 
there  are  two  or  more  strata  of  clouds  one  over  the  other.  Such  is  also 
the  case  with  the  formation  of  hail,  though  there  is  a  difference  in  the 
respective  physical  conditions  of  the  clouds. 

Hail  occurs  during  a  thunder-storm,  when  the  temperature  is  very 
high  upon  the  surface  of  the  ground,  but  decreases  rapidly  with  ele- 
vation. This  rapid  decrease  is  the  principal  element  in  the  formation 
of  hail,  and  it  has  been  known  to  be  as  much  as  one  degree  in  a  little 
more  than  100  feet.  What  then  takes  place  in  the  region  of  clouds  ? 
Those  above,  from  10,000  to  20,000  and  25,000  feet  high,  contain,  the 
highest  of  them,  ice  at  —22°  or  at  —40°  Fahr.;  the  lowest  of  them, 
vesicular  water  at  +14°  and  at  —4°.  The  lower  clouds  contain  vesic- 
ular water  above  32°.  As  a  rule,  these  clouds  travel  in  different  direc- 
tions, and  hail  is  formed  when  there  is  a  collision  and  admixture  of 
winds  that  are  opposed  to  currents  and  clouds  the  temperatures  of 
which  are  different.  The  vapor,  which  then  resolves  into  rain,  freezes 
instantaneously  in  so  cold  a  temperature.  Carried  off  by  the  wind,  and 
.even  exposed  to  the  influence  of  opposite  electricities  of  the  diverse 


HAIL. 


strata  of  cloud,  these  frozen  drops  do  not  fall  at  once,  notwithstanding 
their  weight,  and  they  have  time  to  become  enlarged  by  the  addition 
of  a  considerable  quantity  of  water  which  they  collect  during  their  pas- 
sage through  the  air. 

The  extreme  cold  that  prevails  in  the  clouds  below  the  region  of 
perpetual  snow  is  due,  in  a  great  measure,  to  evaporation,  which  has 
itself  a  double  cause  —  the  action  of  the  sun  and  of  electricity  —  it  hav- 
ing been  remarked  that  after  every  electric  discharge  the  rain  or  hail 
falls  in  great  quantities,  and  the  reaction  produces  a  dilatation  which 
gives  rise  to  rapid  evaporation. 

The  formation  of  hailstones  is  always  a  very  speedy  process.  Volta 
was  of  opinion  that  the  upper  cloud  was  formed  by  the  condensation 
of  vapor  from  the  lower  strata,  and  that  it  contained  positive  electricity, 
while  the  latter  retained  negative  electricity.  Just  as  pith-balls  placed 
between  two  copper  plates  laden  with  opposite  electricity  are  seen  to 
bob  up  and  down  under  the  influences  of  this  double  attraction,  in  the 
same  way  he  thought  that  hail  was  formed  by  a  like  movement  of  the 
corpuscles  of  ice  or  snow,  becoming  successively  enlarged  by  condensed 
vapors.  This  theory  is  not  now  considered  admissible,  and  it  is,  indeed, 
far  simpler  to  suppose  that  hail  is  formed  like  rain,  but  amidst  an  at- 
mospheric cold  which  freezes  the  globules  of  water  at  the  very  moment 
of  their  formation. 

It  appears  that  this  formation,  or  the  shock  of  hailstones  that  are 
borne  along  by  the  wind,  sometimes  produces  a  noise  audible  upon  the 
surface  of  the  ground.  Aristotle  and  Lucretius,  of  ancient  writers,  re- 
cord this  fact;  and  the  meteorologists  Kalm  and  Tessier  assert  that 
they  heard  it,  the  former  in  France  on  July  13,  1788,  the  latter  at  Mos- 
cow on  the  30th  of  April,  1744.  Peltier  states  that  at  Ham  the  ap- 
proach of  a  hailstorm  was  preceded  by  a  sound  like  that  of  a  cavalry 
squadron  at  full  gallop.  In  1871,  M.  Pessot,  corresponding  member  of 
the  Montsouris  Observatory,  reported  from  Doulevant-le-Chateau  (Haute- 
Marne)  a  hailstorm  which  was  preceded  by  this  same  phenomenon. 

The  surfaces  of  hail-clouds  show  here  and  there  immense  irregular 
protuberances.  Seen  from  underneath,  they  are  generally  dark  in  col- 
or, because  of  their  opaqueness,  which  the  solar  light  is  scarcely  able  to 
traverse.  Arago  pointed  out  that  they  seem  to  be  thick,  and  to  be  dis- 
tinguishable from  other  storm-clouds  by  their  ashen  hue.  Their  edges 
are  indented  ;  but  they  very  soon  are  lost  in  the  general  mass  of  the 
nimbi  which  discharge  rain. 


392  THE  ATMOSPHERE. 

To  what  height  do  they  soar?  From  what  elevation  do  hailstones 
fall  ?  Saussure  noticed  a  hailstorm  upon  the  Col  du  Geant  at  a  height 
of  11,246  feet,  Balmat  upon  the  summit  of  Mont  Blanc  itself,  and  Pac- 
card  discovered  hailstones  beneath  the  snow  which  forms  its  peak. 
Hail  often  falls  upon  the  high  slopes  of  the  Alps.  Thus  the  phenome- 
non of  hail  occurs  at  all  elevations.  But  when  the  height  at  which  it 
commences  to  fall  is  very  great,  the  hailstones  melt  during  their  pas- 
sage through  the  thousands  of  feet  of  air  above  the  temperature  of  32° 
which  cover  the  surface  of  the  globe.  In  the  case  of  our  hailstorms, 
on  the  contrary,  the  clouds  which  emit  them  are  at  a  less  height,  and 
seem  to  be  between  5000  and  6500  feet  above  the  ground.  Below 
them  extend  the  storm  and  the  rain  clouds,  at  a  height  of  about  3300 
feet,  or  even  lower.  The  clouds  which  discharge  hail  are  never  very 
large.  Borne  along  by  the  wind,  they  cover  a  narrow  strip  of  land, 
which  is  often  only  three-fifths  of  a  mile  in  breadth,  and  rarely  more 
than  ten  miles  long;  but  the  length  is  sometimes  as  much  as  500  miles. 

One  of  the  most  curious  and  remarkable  hailstorms  in  the  annals  of 
meteorology  is  that  of  July  13, 1788.  It  was  divided  into  two  bands : 
that  on  the  left,  or  the  western  one,  began  at  Touraine,  near  Loches,  at 
6'30  A.M.  ;  passed  over  Chartres  at  7'30  A.M.  ;  over  Rambouillet  at  8 
A.M.  ;  Pontoise,  8'30  A.M.;  Clermont  (Oise)  at  9  A.M.;  Douai,  11  A.M.; 
whence  it  entered  Belgium,  passing  over  Courtrai  at  12'30  P.M. ;  and 
finally  dying  out  beyond  Flushing  at  1430  P.M.  The  total  length  was 
420  miles,  and  it  extended  over  a  width  of  ten  miles. 

The  right,  or  eastern,  branch  began  at  Orleans  at  7*30  A.M.,  passing 
over  Arthenay  and  Andonville,  reached  the  Faubourg  St.  Antoine  in 
Paris  at  8'30  A.M.,  Crepy-en-Valois  at  9'30  A.M.,  Cateau-Cambresis  at  11 
A.M.,  and  Utrecht  at  2'30  P.M.,  the  length  being  near  500  miles,  and  the 
width  only  five  miles.  There  was  a  mean  interval  of  twelve  miles  of 
ground  between  the  two  bands,  and  rain  fell  in  this  space.  The  pas- 
sage of  the  hailstorm  was  preceded  on  each  line  by  a  profound  dark- 
ness. The  speed  of  the  storm  was  thirty-two  miles  per  hour  on  both 
lines,  the  hail  not  falling  for  more  than  seven  or  eight  minutes  in  the 
same  place,  but  with  so  much  violence  that  the  crops  were  cut  to  pieces. 
This  is  the  greatest  hailstorm  known.  No  less  than  1039  communes  in 
France  suffered  from  its  ravages;  the  destruction  of  property  was  found 
to  amount  to  no  less  than  £1,000,000.  The  hailstones  were  not  all  of 
the  same  shape;  some  were  round,  others  long  and  pointed;  and  some 
were  found  to  weigh  3900  grains,  or  more  than  half  a  pound. 


HAIL. 


It  is  seldom  that  the  same  hailstorm  extends  over  such  a  length  of 
country,  and  in  so  regular  a  line.  It  is  probable  that  the  clouds  which 
produced  this  hail  were  more  than  half  a  mile  high.  Generally  they 
are  at  a  less  height  than  this,  and  are  influenced  by  the  undulations  of 
the  soil.  Certain  storms,  without  having  extended  over  so  much 
ground,  are  remarkable  for  their  abundant  quantity.  On  May  9,  1865, 
for  instance,  a  storm  began  at  8'30  A.M.  over  Bordeaux  and  proceeded 
in  a  N.N.E.  direction,  passing  over  Perigueux  at  10  A.M.,  Limoges  at 
noon,  Bourges  at  2  P.M.,  Orleans  at  5'30  P.M.,  Paris  at  7'45  P.M.,  Laon  at 
11  P.M.,  and  collapsing  a  little  after  midnight  in  Belgium  and  the  North 
Sea.  Its  mean  breadth  was  from  fifteen  to  twenty  leagues.  The  hail 
fell  only  in  certain  places:  to  the  left  of  Pe'rigueux,  over  the  arron- 
dissement  of  Limoges,  to  the  right  of  Chateauroux,  to  the  south-east  of 
Paris,  from  Corbeil  to  Lagny,  and  in  the  arrondissements  of  Soissons 
and  Saint  Quentin.  At  this  latter  point  it  was  of  a  formidable  charac- 
ter. The  crystal  mas*  which  fell  from  the  sky  upon  the  Catelet  mead- 
ows formed  a  bed  a  mile  and  a  quarter  long  and  2000  feet  broad,  esti- 
mated to  amount  altogether  to  21,000,000  of  cubic  feet.  The  hailstones 
did  not  disappear  for  more  than  four  days  afterward.  These  hailstones 
sometimes  destroy  all  the  crops,  as,  for  instance,  that  which  occurred  in 
the  neighborhood  of  Angouleme  on  August  3,  1813.  The  day  had 
been  fine,  and  the  wind  was  due  north  until  3  P.M.,  when  it  suddenly 
veered  right  round ;  the  sky  gradually  became  covered  with  clouds, 
which,  collecting  one  on  the  top  of  the  other,  offered  a  terrible  specta- 
cle. The  wind,  which  from  noon  until  5  P.M.  had  been  rather  violent, 
suddenly  dropped.  Thunder  was  heard  in  the  distance,  and  gradually 
became  louder ;  the  sky,  at  last,  became  totally  obscured,  and  at  6  P.M. 
there  was  a  tremendous  fall  of  hail,  the  stones  being  as  large  as  eggs. 
Several  persons  were  severely  wounded,  and  a  child  was  killed  near 
Barbezieux.  The  next  day  the  ground  looked  as  it  might  do  in  mid- 
winter :  the  hailstones  had  accumulated  in  the  hollows  and  the  roads  to 
a  height  of  thirty  to  forty  inches ;  trees  were  entirely  stripped  of  their 
leaves ;  vines  were  cut  into  pieces,  the  crops  crushed,  the  cattle,  sheep 
and  pigs  especially,  were  severely  injured.  The  whole  neighborhood 
was  deprived  of  game,  and  some  few  young  wolves  were  found  dead. 
The  effects  of  the  storm  were  still  visible  in  1818,  the  vines,  in  particu- 
lar, not  having  recovered  their  productive  powers. 

The  storm  which  burst  over  Chaumont,  in  the  Haute  Marne,  on  July 
17. 1852,  spread  over  a  district  nearly  sixty  miles  long  by  five  miles 


394:  THE  ATMOSPHERE. 

broad;  wheat,  vines,  and  nearly  every  tree,  were  destroyed  by  hail- 
stones of  abnormal  dimensions.  The  same  hurricane  swept  violently 
over  the  department  of  the  Aisne,  uprooting  trees,  blowing  down  cot- 
tages, and  killing  several  persons.  In  a  few  seconds  all  trace  of  the 
crops  had  disappeared  from  the  fields. 

On  July  17, 1868,  at  about  8  P.M.,  a  heavy  hailstorm  devastated  the 
neighborhood  of  Rheims;  the  stones  were  as  large  as  Barcelona  nuts, 
and  the  downfall  lasted  three  parts  of  an  hour.  In  some  of  the  hollows, 
where  the  ground  was  sandy,  there  were  remarked  impressions  like 
those  which  might  be  made  by  a  cannon-ball.  These  cavities,  into 
which  the  hailstones  were  first  driven,  constitute  regular  physical  im- 
pressions of  the  hail,  which  seemed,  in  regard  to  the  construction  placed 
by  geologists  upon  similar  marks,  to  possess  a  special  importance. 

Disastrous  hailstorms  are,  fortunately,  rare  in  our  climates,  though 
they  do,  from  time  to  time,  remind  us  of  their  existence.  A  heavy 
storm  began  at  Brussels  on  June  18,  1839,  about  7  P.M.  ;  thick  clouds 
drove  from  the  S.S.W.,  while  at  the  same  time  the  vane  indicated  a 
lower  current  from  the  N.W.  Until  7'30  there  was  a  continuous  roll- 
ing sound,  during  which  the  flashes  of  lightning  succeeded  each  other 
with  astonishing  rapidity.  Soon  after,  a  large  cloud  of  very  ashen 
hue,  and  the  direction  of  which  was  from  W.N.W.  to  S.E.,  veiled  the 
city  in  the  most  complete  obscurity,  and  burst  in  a  shower  of  hail 
which  did  immense  damage.  Most  of  the  hailstones  were  from  a  half 
to  three-quarters  of  an  inch  in  size,  some  of  them  as  much  as  one  inch. 
In  shape  some  were  spherical,  but  the  greater  number  were  more  or  less 
flat.  The  depth  of  water  that  fell  during  the  storm  was  an  inch  and  a 
half.  The  temperature  rose  as  high  as  92°  Fahr. — the  maximum  re- 
corded at  Brussels ;  the  barometer  reading  was  2970  inches  at  4  P.M. 

Hailstorms  have  a  tendency  to  follow  the  direction  of  the  valleys  and 
the  rivers  when  the  clouds  are  not  high ;  for,  as  is  shown  by  the  cases 
cited  above,  the  storms  then  become  regular  currents,  which  come  from 
the  Atlantic  and,  following  the  ordinary  course  of  the  currents  which 
reach  us,  continue  their  progress  from  the  south-westerly  regions  to- 
ward those  of  the  north-east.  But  in  all  partial  secondary  storms  (which 
are  the  most  frequent,  and  are  generally  confined  to  a  limited  area)  there 
is  an  evident  deviation  from  the  valleys.  It  seems,  too,  that  they  keep 
away  from  forests.  Since  meteorological  facts  have  been  registered  by 
the  French  Ecoles  Normales,  there  has  been  plenty  of  evidence  collected 
as  to  the  influence  of  the  ground  in  regard  to  the  distribution  of  storms 


HAIL. 


and  of  hail.  One  district  may  be  visited  by  hailstorms  every  year,  an- 
other not  once  in  ten  years.  It  has  even  been  found  possible  to  com- 
pose statistical  maps  showing  the  damage  done  by  the  hail  in  each  de- 
partment, by  aid  of  the  documents  appertaining  to  insurance  companies. 
These  maps  are  scarcely  reliable  from  a  meteorological  point  of  view 
as  they  are  based  on  pecuniary  losses  ;  and  the  same  quantity  of  hail 
would  cause  ten  times  as  much  damage  were  it  to  fall  over  a  tobacco 
plantation  of  the  Lower  Ehine,  as  it  would  if  it  were  to  rage  over  an 
uncultivated  or  even  a  wooded  district.  It  is  true  that  the  intrinsic 
quantity  of  hail  differs  in  neighboring  countries,  according  to  their  geo- 
logical, orographical,  and  climatological  situation. 

Hailstorms  are  those  in  which  the  development  of  electrity  attains  the 
largest  proportions.  The  thick  clouds  in  which  the  meteor  becomes 
elaborated  are  laden  with  a  large  quantity  of  the  electrical  fluid,  part  of 
which  becomes  exhausted  within  themselves  or  in  reciprocal  discharges 
with  neighboring  clouds. 

The  thunder  is  then  not  merely  a  report  following  the  flash  ;  it  is  a 
continuous  rolling  sound,  during  which  it  is  not  unusual  for  no  light- 
ning to  be  perceptible,  either  because  the  flashes  are  of  very  small  di- 
mensions or  because  they  take  place  entirely  within  the  interior  of  the 
clouds.  Thus,  on  the  4th  of  September,  1871,  1  noticed,  in  the  hail- 
storm which  took  place  in  Paris  at  3'36  P.M.,  that  when  the  hail  had 
passed  over  the  district  in  which  the  Observatory  is  situated,  and  when 
it  was  over  Menilmontant,  there  was  a  continuous,  rolling  of  thunder, 
unaccompanied  by  lightning,  which  lasted  six  minutes,  and  recommenced 
again  after  several  short  intervals.  On  the  7th  of  May,  1865,  a  violent 
storm  burst  over  the  department  of  the  Aisne,  causing  damages  amount- 
ing to  several  million  francs.  Above  the  strata  of  clouds  there  was 
visible  a  thick  cumulus,  of  a  livid  white  hue,  from  which  there  was  a 
continuous  flashing  of  lightning  ;  the  rolling  of  the  thunder  was  unin- 
terrupted, though  not  very  loud  ;  there  was  an  unintermittent  crepita- 
tion of  the  lightning,  and  the  explosions  seemed  to  be  confiped  to  the 
interior  of  the  largest  cloud.  When  the  cloud  had  slowly  ascended  the 
heights  of  Roussay,  upon  the  apex  of  the  basins  of  the  Somme  and  the 
Scheldt,  it  swept  down  with  tremendous  rapidity  into  the  valley  of  this 
latter  stream,  pelting  Vend'huile,  Catelet,  and  Beaurevoir,  with  so  many 
hailstones  that  they  lay  five  yards  deep  upon  the  ground.  They  were 
still  visible  five  days  after,  and,  at  some  places,  formed  such  a  solid  mass 
that  they  acted  as  a  dike  to  keep  back  the  water.  When  it  was  at- 


396  THE  ATMOSPHERE. 

tempted  to  sweep  them  away  they  slipped  along  like  fields  of  ice !  M. 
Quetelet  remarked,  during  a  severe  storm  that  occurred  at  Brussels  on 
June  18,  1839,  a  continuous  rolling  of  thunder,  during  which  time  the 
flashes  of  lightning  succeeded  each  other  with  marvelous  rapidity.  Soon 
after,  a  thick  ashen  cloud  plunged  the  whole  city  into  profound  dark- 
ness, and  burst  in  a  heavy  fall  of  hail. 

It  is  interesting  to  ascertain  what  is  the  greatest  dimension  which  a 
hailstone  can  attain.  I  am  able  to  give  some  very  curious  comparisons 
on  this  subject  from  a  number  of  well-authenticated  documents. 

After  the  great  hailstorm  of  July  13, 1788,  alluded  to  above,  the  geol- 
ogist Tessier  cut  pieces  of  ice  which  seemed  to  him  to  be  of  the  con- 
sistency of  hail,  into  the  shape  and  size  of  pigeons',  hens',  and  turkeys' 
eggs,  in  order  that  meteorologists  might  be  enabled  to  calculate  approx- 
imately the  weight  of  hailstones  according  to  their  size.  The  first 
weighed  169  grains;  the  second,  254;  and  the  third,  1065  grains. 

The  most  ordinary  size  of  a  hailstone  is  that  of  a  small  nut:  some, 
indeed,  are  not  larger  than  a  good-sized  pea.  In  ordinary  storms,  the 
stones  weigh  from  46  to  120  grains. 

The  three  weights  above  often  occur  in  the  annals  of  meteorology. 
There  is  nothing  absolutely  abnormal  in  a  fall  of  hailstones  weighing 
from  a  quarter  of  an  ounce  to  two  and  a  quarter  ounces. 

Some  extraordinary  facts  are  the  following,  which  are,  however,  per- 
fectly authenticated  and  certified  by  well-known  -savants :  In  a  disastrous 
hailstorm  near  the  Ehine,  a  hailstone  was  picked  up  by  Voget  at  Heins- 
berg  weighing  1400  grains.  At  Eanderath  they  weighed  twice  as  much. 
During  a  storm  that  occurred  at  Morbihan,  and  which  lasted  three-quar- 
ters of  an  hour,  on  June  21, 1846,  the  hailstones  were  of  all  dimensions, 
from  the  size  of  a  nut  to  that  of  a  turkey's  egg.  One  was  eight  and 
three-quarter  inches  in  circumference.  Muncke  weighed  some  hailstones 
in  Hainault  that  exceeded  three  and  three-quarter  ounces  in  weight. 
Halley  relates  that  some  hailstones  were  picked  up  on  April  29,  1697, 
in  Flintshire,  the  weight  of  which  exceeded  four  ounces ;  and  on  May 
4,  in  the  same  year,  Taylor  found  that  the  circumference  of  some  that 
fell  in  Staffordshire  was  eleven  and  three-quarter  inches. 

Yolney  tells  us  how,  during  the  storm  of  July  13,  1788,  he  was  stay- 
ing at  Pontchartrain,  ten  miles  from  Versailles.  The  sun's  rays  were 
almost  unbearable  ;  the  air  still  and  suffocating;  the  sky  was  cloudless, 
and  claps  of  thunder  were  from  time  to  time  audible.  Toward  7'15 
P.M.  a  cloud  appeared  in  the  south-west,  followed  by  a  very  sharp  wind. 


HAIL. 


397 

"A  few  minutes  afterward  the  cloud  filled  the  horizon  and  sped  toward 
our  zenith,  accompanied  by  a  wind  which  had  become  quite  cool;  hail 
began  to  fall  obliquely  at  an  angle  of  45°,  the  stones  being  as  large  as 
pieces  of  plaster  thrown  down  from  the  top  of  a  house.  I  could  scarce- 
ly believe  my  eyes ;  several  of  the  stones  were  as  large  as  a  man's  fist, 
and  some  of  these  were  but  pieces  that  had  been  broken  off  stones  still 
larger.  When  I  ventured  to  put  out  my  hand  beyond  the  door  of  the 
house  where  I  had  taken  refuge,  I  picked  up  one  and  found  that  it 
weighed  more  than  five  ounces.  It  was  very  irregular  in  shape,  there 
being  three  protuberances,  thick  as  the  thumb  and  nearly  as  long, 
which  projected  from  the  main  body  of  the  stone !" 

Volta  states  that,  during  the  night  of  April  19-20, 1787,  among  the 
enormous  hailstones  which  fell  in  Como  and  the  neighborhood,  there 
was  one  which  weighed  nearly  nine  ounces.  Parent,  member  of  the 
Academy  of  Sciences,  relates  that  hailstones  as  big  as  a  man's  fist,  and 
weighing  from  nine  and  a  half  ounces  to  twelve  and  three-quarter 
ounces,  fell  in  Le  Perche  on  May  15,  1703.  Montignot  and  Tressan 
picked  up  some  at  Toul  on  July  11,  1753,  which  had  the  shape  of  an 
irregular  polyhedron,  with  a  diameter  of  three  inches. 

During  a  hailstorm  at  Constantinople  on  October  5, 1831,  there  fell 
stones  weighing  more  than  one  pound,  and  -larger  than  a  man's  fist. 
Analogous  stones  are  said  to  have  been  picked  up  in  May,  1821,  at 
Palestrina  (Italy). 

The  following  are,  however,  even  more  remarkable  instances:  On 
June  15,  1829,  there  was  a  hailstorm  at  Cazorta,  in  Spain,  which  crushed 
in  houses ;  some  of  the  blocks  of  ice  weighed  four  and  a  half  pounds. 
For  hailstones  to  attain  such  proportions,  several  must  have  become 
agglomerated  together,  either  when  they  reached  the  ground  or  during 
their  descent.  This  is,  in  fact,  in  accordance  with  experience.  And 
this  explanation  is,  therefore,  specially  applicable  to  the  following  cases, 
if,  indeed,  they  be  authentic:  During  the  latter  part  of  October,  1844, 
during  a  terrible  hurricane  which  devastated  the  south  of  France,  there 
fell  hailstones  weighing  eleven  pounds;  the  town  of  Cette,  in  particular, 
was  severely  damaged ;  men  were  struck  to  the  ground  as  if  they  had 
been  stoned,  partition  walls  were  blown  down,  and  vessels  sunk. 

It  seems  that  there  was  a  very  singular  hailstorm  on  May  8,  1802,  a 
piece  of  ice  having  been  picked  up  which  measured  more  than  three  feet 
both  in  length  and  in  width,  with  a  thickness  of  two  and  a  quarter  feet. 
Dr.  Foissac,  who  cites  this  fact,  does  not  consider  it  to  be  an  exaggera- 


398  THE  ATMOSPHERE.  . 

tion ;  and  he  adds,  "M.  Hue,  a  Catholic  missionary  in  Tartary,  relates 
that  hailstones  of  a  remarkable  size  often  fall  in  Mongolia,  and  that  some 
of  them  have  been  found  to  weigh  twelve  pounds.  During  a  heavy 
storm  in  1843  the  noise  as  of  a  terrible  wind  was  heard  in  the  air,  and 
soon  after  there  fell  in  a  field  not  far  from  our  house  a  piece  of  ice  larger 
than  a  millstone.  It  was  broken  up  with  a  hatchet;  and  though  the 
weather  was  very  warm,  it  took  three  days  to  melt  completely." 


Fig.  T3.— Section  of  hailstones,  showing  their  ordinary  interior  structure. 

If  this  be  true,  there  is  nothing  improbable  in  the  chronicle  dating 
from  Charlemagne,  which  relates  that  there  fell  hailstones  fifteen  feet 
wide  by  six  long  and  eleven  thick,  nor  in  that  of  Tippoo  Sahib,  which 
speaks  of  a  hailstone  as  big  as  an  elephant. 

The  shape  of  hailstones  differs  very  much.  They  are,  as  a  rule, 
round,  spherical,  more  or  less  irregular,  like  peas,  grapes,  or  nuts.  Sev- 
eral are  more  elongated,  like  a  grain  of  wheat,  cornelian  cherries,  or 
olives.  When  very  large,  they  are  formed  by  the  juxtaposition  of 
crystallized  particles.  On  July  4,  1819,  during  a  nocturnal  storm  which 
spread  over  a  large  portion  of  Western  France,  Delcros  picked  up  sev- 
eral entire  spherical  hailstones,  in  which  was  visible  a  first  spherical 
nucleus  of  a  somewhat  opaque,  whitish  hue,  offering  the  traces  of  con- 
centric strata.  Around  this  nucleus  was  an  envelope  of  compact  ice, 
radiated  from  the  centre  to  the  circumference,  and  terminating  upon 
the  exterior  with  twelve  large  pyramids,  between  which  were  inter- 
calated smaller  pyramids.  The  whole  formed  a  spherical  mass  nearly 
three  and  a  half  inches  in  diameter. 

Some  hailstones  picked  up  on  September  12,  1863,  in  a  road  to  the 
south-west  of  Tiflis,  drawings  of  which  were  exhibited  to  the  Acadenry 
of  Sciences  at  St.  Petersburg,  were  ellipsoidal  in  shape,  and  their  sur- 


HAIL.  399 

face  was  covered  with  a  large  number  of  small  prominences.  The  poly- 
hedric  tissue,  examined  through 
a  glass,  had  the  aspect  of  a  series 
of  six -fronted  pyramids;  and  a 
section  of  the  interior  revealed 
the  existence  of  a  hexagonal  net- 
work of  meshes,  which  is  repre- 
sented in  Fig.  74. 

On  July  29,  1871,  at  6  P.M.,  the 
sun  shining  brightly,  and  there 
being  hardly  any  clouds,  a  sound 
was  heard  at  Auxerre,  like  that 
of  a  heavy  luggage-train.  A  few 
flashes  of  lightning  preceded  the 
fall  of  the  hail,  which  came  down  Fig"  74-Section  of  a  h»il8tone'  «"•**. 
unaccompanied  by  any  tempest  or  atmospheric  disturbance.  The  hail- 
stones preserved  their  shapes  when  they  reached  the  ground,  which  are 
represented  in  the  four  corners  of  Fig.  74,  after  the  designs  of  M. 
Daudin.  The  two  stones  in  the  centre  are  those  to  which  I  alluded  in 
connection  with  the  Academy  of  St.  Petersburg,  and  the  remainder  have 
been  added  as  illustrative  of  the  smaller  and  more  usual  size  of  hail- 
stones. During  the  same  storm  M.  Parent  remarked  at  Montargis  that 
there  was  a  heavy  fall  of  hail  at  6'45  P.M.,  the  pieces  of  ice  being  from 
one  to  two  inches  in  length,  oval  in  shape,  and  transparent  as  crystal. 

During  the  storm  of  May  22,  1870,  in  Paris,  M.  Tre'cul,  of  the  Insti- 
tute, noticed  that  several  of  the  hailstones  were  conical,  or  rather  pyri- 
form — that  is,  larger  at  the  base  than  at  the  top,  some  of  them  being 
about  three-quarters  of  an  inch  long  by  half  an  inch  wide.  One  of 
them,  carefully  examined,  presented  characteristics  worthy  of  notice. 
The  third  part  of  it,  at  the  top  (the  narrowest  portion  of  the  hailstone), 
was  opaque  and  white;  while  the  lower,  or  the  broadest,  part  was  per- 
fectly translucid,  like  the  purest  ice.  In  addition,  this  hailstone,  when 
looked  at  from  its  broadest  end— that  is,  when  the  narrowest  diameter 
was  placed  crosswise  in  respect  to  the  visual  axis— presented  the  shape 
of  an  obtuse-angled  rhombus ;  and  from  the  sides  there  started  oblique 
facets  which  converged  and  died  away  toward  the  obtuse  summit  of  the 
hailstone. 

As  to  the  epochs  of  hailstorms,  it  is  generally  known  that  they  occur 
in  summer  and  in  the  afternoon— that  is,  when  the  meteorological  con- 
ditions mentioned  above  happen  together— viz.,  great  heat  upon  the 


400 


THE  ATMOSPHERE. 


K.  75.— Different  forms  of  hail. 


surface  of  the  ground,  which  diminishes  rapidly  with  increase  of  eleva- 
tion, and  which  is  accompanied  by  a  considerable  evaporation  from  the 
clouds  under  the  action  of  the  sun.  As,  however,  the  mere  collision  of 
a  very  cold  upper  wind  with  a  very  warm  wind  at  the  same  altitude  may 
produce  hail,  it  occasionally  falls  in  winter  and  at  night;  but  this  is  of 
rare  occurrence. 

Meteorologists  often  class  together  hoar-frost  and  hail,  and  hence  as- 
sert that  these  aqueous  meteors  occur  oftener  in  winter  and  spring  than 
in  summer  and  autumn.  But  hoar-frost  differs  from  hail,  not  only 
from  being  divided  into  so  much  smaller  particles,  but  in  its  mode  of 
formation,  for  it  does  not  spring  from  the  bosom  of  the  clouds,  nor  does 
it  necessitate  great  atmospheric  movements.  It  is  merely  frozen  rain. 
or  a  rough-grained  and  dense  snow. 


PKODIGIES. 


CHAPTER  V. 
PRODIGIES:    SHOWERS   OF  BLOOD  — or  EARTH  — OF   SULPHUR -OF 

PLANTS— OF  FROGS— OF  FISH— OF  VARIOUS  KINDS  OF  ANIMALS. 

APART  from  the  ordinary  showers,  more  or  less  heavy,  of  rain,  snow, 
or  hail,  which  we  have  been  considering  above,  the  history  of  meteors 
is  supplemented  by  certain  extraordinary  showers  which  have  often  in- 
spired the  ignorant  and  credulous  with  terror,  who  have  seen  in  them 
direct  manifestations  of  God's  anger. 

I  do  not  refer  to  stones  falling  from  the  sky,  the  aerolites,  which 
Greek  philosophers  looked  upon  as  fragments  detached  from  the  celes- 
tial vault,  but  which  are,  as  we  have  seen,  cosmical  corpuscles  circu- 
lating in  space.  Nor  will  we  deal  with  the  showers  of  stones,  bricks, 
planks,  and  earthenware,  which  are  caused  by  whirlwinds.  But  we  will 
just  glance  at  certain  phenomena  which  we  have  not  yet  taken  notice 
of.  We  will  begin  by  the  Showers  of  Blood. 

Homer  relates  how  a  shower  of  blood  fell  upon  the  heroes  of  Greece, 
as  a  presage  of  death  for  many  of  their  number.  Obsequens  cites  the 
following :  After  the  capture  of  Fidenes,  in  the  year  14  of  the  Romish 
era,  drops  of  blood  fell  from  the  sky,  to  the  great  surprise  of  all  men. 
In  538  a  heavy  shower  of  blood  fell  over  the  Aventine  Hill  and  at 
Aricia.  In  570  and  572  it  rained  blood  for  two  days  upon  the  Squares 
of  Vulcan  and  Concordia;  in  585  during  one  day.  In  587  this  prodi- 
gy occurred  in  several  districts  of  the  Campagna,  upon  the  territory  of 
Preeneste ;  in  626  at  Ceres,  in  648  at  Rome,  in  650  at  Duna,  in  652  in 
the  neighborhood  of  the  Anio.  There  was  a  shower  of  blood  when 
Tatius  was  murdered.  Plutarch  speaks  of  showers  of  blood  after  great 
battles — in  the  Cimbric  war,  for  instance,  after  the  massacre  of  so  many 
thousand  Cimbri  upon  the  plains  of  Marseilles.  He  admits  that  the 
bloody  vapors  distilled  from  the  corpses  and  diluted  in  the  clouds 
would  lend  to  these  their  crimson  inge.  The  following  are  the  show- 
ers of  blood  which,  principally  by  aid  of  the  researches  made  by  M. 
Grellois,  I  have  succeeded'  in  collecting  as  having  occurred  since  the 
commencement  of  the  Christian  era  down  to  the  close  of  the  last  cen- 
tury. In  the  first  instance,  Gregory  of  Tours  relates  that  in  the  year 

26 


402  THE  ATMOSPHERE. 

582  A.D.  "  a  shower  of  blood  fell  over  the  district  about  Paris.  Many 
persons  had  their  clothes  stained  with  it,  and  cast  them  off  in  terror." 
An  analogous  shower  is  said  to  have  taken  place  at  Constantinople  in 
652.  In  654  the  sky  seemed  on  fire  in  Gaul,  blood  descending  from 
the  clouds  in  large  quantities.  In  787  Fritsch  mentions  a  shower  of 
blood  in  Hungary,  followed  by  the  plague.  Others  were  witnessed  at 
Brixen  in  869,  and  at  Bagdad  in  929.  In  1117  there  occurred  strange 
phenomena,  showers  of  blood,  and  subterraneous  noises,  which  scat- 
tered terror  throughout  Lornbardy  during  the  struggle  for  freedom 
there,  and  a  meeting  of  Bishops  took  place  at  Milan  to  consider  their 
origin.  The  same  phenomenon  was  remarked  at  Brescia  for  three 
days  and  three  nights  before  the  death  of  the  Pope,  Adrian  II.  In 
1144  there  were  several  showers  of  blood  in  Germany ;  in  1163  at  La 
Eochelle.  In  1181,  during  the  month  of  March,  there  was  a  constant 
rain  of  blood  for  three  days  in  France  and  Germany :  a  luminous  cross 
was  visible  in  the  skies.  Toward  the  end  of  1543  blood  fell  at  the  cas- 
tle of  Sassemburg,  near  Barendorf,  in  Westphalia ;  in  1580  at  Louvain. 
In  1571  there  fell  near  Einden,  during  the  night,  so  much  blood  that 
over  a  space  of  five  or  six  miles  the  grass  and  clothes  exposed  had 
assumed  a  dark  purple  hue.  Many  persons  preserved  some  of  it  in 
vessels.  It  was  attempted,  but  unsuccessfully,  to  show  that  this  prodigy 
was  due  to  the  rising  into  the  air  of  the  vapor  from  the  blood  of  oxen 
that  had  been  killed.  No  other  explanation  was  found  more  deserving 
of  credit  among  natural  causes.  These  phenomena  were  also  noticed 
at  Strasbourg  in  1623,  at  Tournay  in  1638,  and  at  Brussels  in  1640. 

We  learn  from  the  records  of  the  Academy  of  Sciences  that  on 
March  17, 1669,  at  4  A.M.,  there  fell  in  several  parts  of  the  town  of 
Chatillon-sur-Seine  a  kind  of  rain  or  reddish  liquor,  thick,  viscous,  and 
putrid,  which  resembled  a  shower  of  blood.  Large  drops  were  seen 
imprinted  against  walls,  and  one  wall  was  even  splashed  all  over  on 
both  sides,  "  which  would  lead  one  to  believe  that  this  rain  was  com- 
posed of  stagnant  and  muddy  waters,  carried  into  the  air  by  a  hurricane 
out  of  some  neighboring  marshes."  There  was  a  shower  of  blood  at 
Venice  in  1689. 

In  1744  there  fell  a  red  rain  in  the  Faubourg  of  St.  Peter  d' Arena, 
at  Genoa,  which,  on  account  of  the  war  then  going  on  in  the  territory 
of  the  Republic,  terrified  the  inhabitants  very  much  ;  but  it  was  subse- 
quently ascertained  that  this  tint  was  due  to  some  red  earth  which  a 
strong  wind  had  carried  into  the  air  from  a  neighboring  mountain. 


PRODIGIES.  40g 

History  speaks  of  showers  of  blood  at  Cleves  in  1763,  in  Picardy  in 
1765,  and  in  Italy  in  1803.  Eain  of  a  red  color  has  been  observed  oft- 
en enough  in  our  own  day  to  prevent  there  being  any  doubt  as  to  the 
reality  of  the  phenomenon,  and  the  only  mistake  of  our  forefathers  was 
in  assigning  it  a  supernatural  origin.  Bede  was  of  opinion  that  a  rain 
thicker  and  warmer  than  usual  might  become  blood-red,  and  so  deceive 
the  uninstructed.  Kaswini,  El  Hazen,  and  other  savans  of  the  Middle 
Ages,  relate  that  about  the  middle  of  the  ninth  century  there  fell  a  red 
powder  and  a  matter  resembling  coagulated  blood.  These  philosophers 
were-thus  on  the  road  to  a  reasonable  explanation ;  they  saw  in  it  only 
a  resemblance  which  might  be  correct,  and  not  a  reality  which  is  re- 
pugnant to  the  simplest  logic.  "What  the  vulgar  call  a  shower  of 
blood,"  says  Gr.  Schott,  "  is  generally  a  mere  fall  of  vapors  tinted  with 
vermilion  or  red  chalk.  But  when  blood  actually  does  fall,  which  it 
would  be  difficult  to  deny  takes  place,  it  is  a  miracle  due  to  the  will  of 
God."  Eustathius,  the  commentator  of  Homer,  says  that  in  Armenia 
the  clouds  discharge  showers  of  blood  because  this  country  contains  the 
Cinabrian  mines,  the  dust  of  which,  mixed  with  water,  colors  the  drops 
of  rain.  • 

Conrad  Lycosthenes,  in  his  "  Book  upon  Prodigies,"  represents  the 
showers  of  blood  and  the  showers  of  crosses  in  the  shape  of  childish 
figures,  which  give  us  an  idea  of  the  simple-mindedness  prevalent  in 
those  days. 

In  the  early  part  of  July,  1608,  one  of  these  pretended  showers  of 
blood  fell  in  the  outskirts  of  Aix  (Provence),  and  this  shower  extended 
to  the  distance  of  half  a  league  from  the  town.     Some  priests,  either 
being  themselves  deceived  or  wishing  to  work  upon  the  credulity  of 
the  people,  at  once  attributed  it  to  diabolic  influence.     Fortunately,  a 
person  of  education,  M.  de  Peiresc,  examined  very  minutely  into  this 
apparent  prodigy,  studying  in  particular  some  drops  that  fell  upon  the 
wall  of  the  cemetery  attached  to  the  principal  church  in  Aix.    He  soon 
discovered  that  they  were  in  reality  the  excrements  of  some  butterflies 
which  had  been  noticed  in  large  numbers  during  the  early  part  of  July. 
There  were  no  spots  of  the  kind  in  the  centre  of  the  town,  where  the 
butterflies  had  not  made  their  appearance,  and,  moreover,  none  wen 
noticed  upon  the  higher  parts  of  the  houses,  above  the  level  to  whi. 
they  flew.     Besides,  the  presence  of  these  drops  in  places  protect 
from  the  air  rendered  it  impossible  that  they  could  have  their  o 
in  the  atmosphere.     He  at  once  pointed  this  out  to  those  who  regarde 


404 


THE  ATMOSPHERE. 


the  occurrence  as  miraculous ;  but,  in  despite  of  the  proofs  which  he 
adduced,  the  inhabitants  persisted  in  attributing  these  drops  to  a  su- 
pernatural cause. 


Fig.  76.—  Rain  of  blood  iu  Proveuce,  July,  1608. 

Reaumur  gives  the  butterfly  known  as  "the  great  turtle"  as  being 
the  most  capable  of  depositing  these  drops.  "  There  are  thousands  of 
others,"  he  says,  "  which  turn  into  chrysalises  toward  the  end  of  May 
or  the  beginning  of  June.  When  this  transformation  is  about  to  take 
place,  they  leave  the  trees  and  often  take  refuge  upon  walls,  entering 
houses,  hanging  on  to  the  arch  of  a  door-way  or  a  plank.  If  the  but- 
terflies which  emerge  from  them  at  the  end  of  June  or  the  beginning 
of  July  flew  in  masses  together,  they  would  be  numerous  enough  to 
form  small  clouds,  and  consequently  to  cover  the  stones  in  certain 
places  with  spots  of  a  blood-red  color,  and  thus  to  make  the  timid  be- 
lieve that  they  were  spectators  of  a  supernatural  occurrence."  Gen- 


PRODIGIES.  405 

erally  speaking,  showers  of  blood  are  not  only  red  spots  produced  by 
certain  insects,  but  regular  showers,  colored  by  the  dust  which  the  wind 
carries  into  the  air.  This  general  origin  was  not  ascertained  until  the 
present  century.  On  March  14,  .1813,  one  of  these  strange  red  showers 
fell  in  the  kingdom  of  Naples  and  the  Two  Calabrias.  Sementina  ex- 
amined and  analyzed  it,  rendering  the  following  account  to  the  Naples 
Academy  of  Sciences:  "An  east  wind  had  been  blowing  for  two  days, 
when  the  inhabitants  of  Gerace  noticed  a  dense  cloud  moving  toward 
the  sea.  At  2  P.M.  the  sea  became  calm,  but  the  cloud  already  covered 
the  neighboring  mountains  and  began  to  intercept  the  light  of  the  sun. 
Its  color,  originally  a  pale  red,  soon  became  deep  as  fire.  The  town 
was  then  plunged  into  such  profound  darkness  that,  about  4  P.M.,  it 
was  necessary  to  light  candles  in  the  houses.  The  inhabitants,  alarmed 
by  the  obscurity  and  the  color  of  the  cloud,  rushed  in  crowds  to  the 
cathedral  to  pray.  The  obscurity  increased,  and  the  whole  sky  seemed 
red  as  fire;  thunder  began  to  growl ;  and  the  sea,  though  six  miles  dis- 
tant, added  to  the  general  alarm  by  the  roar  of  its  waves.  There  then 
began  to  fall  large  drops  of  reddish  rain,  which  many  persons  took  for 
blood,  and  others  for  fire.  At  last,  as  night  advanced,  the  air  became 
clear,  the  thunder  and  lightning  ceased,  and  the  inhabitants  regained 
their  self-possession." 

With  the  exception  of  there  being  no  popular  alarm,  the  same  phe- 
nomenon of  a  shower  of  reddish  dust  occurred  not  only  in  the  Two 
Calabrias,  but  also  at  the  opposite  extremity  of  the  Abruzzes.  This 
dust  was  of  a  yellowish  hue,  like  cinnamon,  and  had  a  slight  earthy' 
taste ;  it  was  unctuous  to  the  touch,  and,  seen  through  a  glass,  con- 
tained small  and  hard  bodies  resembling  pyroxene.  Heat  at  first 
embrowned  it,  then  made  it  black,  and  finally  gave  it  a  reddish  tint. 
After  the  action  of  the  heat,  this  dust  displayed,,  even  to  the  naked  eye, 
an  immense  number  of  small  and  brilliant  points,  which  were  of  yellow 
mica.  Its  specific  gravity,  when  deprived  of  hard  substances,  was  2'07 : 
it  was  composed  of  silica,  33 0;  aluminium,  15-5;  lime,  11-5;  chrome, 
1-0  ;  iron,  14-5  ;  and  carbonic  acid,  9'0. 

Whence  came  this  dust?  This  it  was  found  impossible  to  ascertain 
at  that  time.  It  was  not  until  1846  that  a  general  examination  of  these 
rains  was  made,  and  their  origin  found  by  following  them  up  into 
space.  On  May  16  in  that  year  an  earthy  rain  fouled  all  the  wat< 
Syam  (Jura).  In  the  autumn  of  the  same  year  there  was  a  similar  fall, 
.  -,  i  i-  V,.  •_„.  ;Ki,,,r;oT,  T-nin  vprv  disastrous  hurricanes. 


accom 


panied   by  lightning,  diluvian  rain,  very  disastrous  hurricanes, 


406  THE  ATMOSPHERE. 

etc.,  which  occurred  alternately,  or  nearly  so,  over  a  large  circular  tract 
of  country,  in  such  a  way  as  to  be  only  explicable  by  some  great  dis- 
turbance in  the  system  of  the  trade-winds.  The  cyclones  also  swept 
over  the  Atlantic;  amidst  fearful  squalls,  whirlwinds,  and  hailstorms, 
vessels  were  dismasted  and  their  decks  swept  clean.  Then  also  oc- 
curred severe  tempests  in  France,  Italy,  and  at  Constantinople ;  while, 
farther  eastward,  the  typhoons  spent  their  fury  in  the  China  seas.  The 
winds  were  sufficiently  intense  to  detach  a  stratum  of  land  in  districts 
where  the  surface  of  the  ground  was  sandy  or  of  some  other  soft  sub- 
stance. This  earth,  carried  into  the  air,  was,  of  course,  certain  to  be 
deposited  somewhere.  This  took  place  in  the  south  of  France,  between 
Puy  and  Mont  Cenis,  in  the  direction  of  the  prevailing  wind,  and  cross- 
wise from  Bourg  to  Drome.  The  quantity  of  earth  precipitated  varied, 
however,  according  to  the  locality ;  at  Lyons,  in  fact,  it  was  scarcely 
apparent,  though  it  occurred  in  the  shape  of  a  reddish  slime  which  was 
popularly  converted  into  a  shower  of  blood.  But  at  Meximieux  a  bat- 
talion of  soldiers  marching  toward  the  Swiss  frontier  were  covered  with 
the  mud,  and  their  uniforms  impregnated  with  it.  The  Chateau  de 
Chamagnieu  was  bespattered  in  such  a  way  that  it  could  scarcely  be 
recognized,  and  there  was  such  a  thick  la}rer  at  Valence  that  the  inhab- 
itants were  compelled  to  clean  water-shoots  and  gutters.  Fournet  gives 
a  calculation  which  shows  that  in  the  department  of  the  Drome  the 
clouds  must  have  taken  up  from  and  again  discharged  upon  the  ground 
the  enormous  weight  of  720  tons,  which  represent  180  four-horse  wag- 
on-loads. Ehrenberg,  who  analyzed  samples  of  this  earth,  found  in 
them  seventy-three  organic  formations,  some  of  which  were  peculiar  to 
Southern  America,  This  earth  must,  therefore,  have  come  from  the 
New  World.  The  interval  of  time  between  their  leaving  America,  Oc- 
tober 13,  and  their  arrival  in  France,  October  17,  was  about  four  days, 
which  gives  a  speed  of  eighteen  and  three-quarter  yards  per  second. 

Subsequent  to  that  date  we  have  had  a  remarkable  fall  of  colored 
rai'n  in  the  neighborhood  of  ChambeVy,  on  March  31, 1847.  It  was 
imbued  with  a  milky  matter,  which  seemed  like  thin  clay  suspended  in 
the  air.  The  clothes  of  persons  exposed  to  this  rain  were  bespattered 
with  whitish  spots.  Information  from  Savoy  and  tlie  Great  St.  Ber- 
nard came  to  hand  soon  after  this,  stating  that  there  had  been  a  fall  of 
earthy  red  snow,  coming  from  the  south-west,  and  covering  the  ground 
to  the  depth  of  several  inches. 

This  coloring  of  the  snow  by  the  dust  must  not  be  confounded  with 


PRODIGIES.  4Q7 

a  hue  which  it  often  derives  from  a  small  insect  which  lives  in  it 

uredo  nivalis — a  kind  of  microscopic  infusory  often  extraordinarily  nu- 
merous in  the  Alps  and  the  Polar  regions. 

At  the  period  of  the  red  rain  in  1847  cited  above,  the  falls  of  snow 
extended  over  a  large  portion  of  France— at  Orleans,  at  Paris,  in  the 
Vosges,  and  La  Bresse  ;  and  there  were  hurricanes  at  Havana,  Bahama, 
the  Azores,  Newfoundland,  the  Sorlingues,  Portugal,  and  Spain.  There 
were  numerous  atmospheric  whirlwinds  in  the  north  and  the  west,  at 
Le  Havre,  Paris,  and  at  Grignan,  no  less  than  twenty-four  storks  fall- 
ing dead  at  this  place.  At  Nantua,  a  whirlwind,  which  carried  a  sen- 
try-box ten  feet  into  the  air,  covered  the  streets  with  debris  of  tiles, 
chimneys,  and  windows.  The  numbers  given  by  Fournet  show  a  very 
rapid  and  marked  depression  of  the  barometer  on  March  31,  followed 
by  a  still  greater  decrease  on  April  2. 

There  was  also  a  very  remarkable  shower  of  earth  on  March  27, 
1862.  The  residue,  when  moist,  was,  like  that  of  1846,  so  far  red  in 
hue  as  to  revive  the  popular  belief  about  a  shower  of  blood  ;  when  dry, 
the  earth  was  fine  and  yellowish.  Ehrenberg  discovered  in  it  forty- 
four  organic  forms,  among  which  were  those  microscopical  galionelles,  a 
cubic  inch  of  which  may.contain  466,000. 

The  shower  which  fell  at  Beauvais  in  May,  1863,  from  5  to  11  A.M., 
was  also  very  remarkable,  the  spots  which  it  left  upon  clothes  being  as 
marked  as  in  the  preceding  cases. 

About  3  A.M.  on  the  morning  of  May  1,  a  violent  thunder-storm 
broke  over  Perpignan,  and  afterward  a  reddish  dust  was  noticed  in 
several  parts  of  the  town,  which,  it  was  subsequently  ascertained,  must 
have  fallen  during  the  storm.  The  same  storm  extended  to  the  level 
district  in  the  department  of  the  Eastern  Pyrenees ;  but  here  the  phe- 
nomenon witnessed  was  a  fall  of  red  snow,  and  the  appearance  of  these 
flakes  alarmed  the  inhabitants.  The  occurrence  was  also  noticed  on 
many  coast-towns  of  the  Mediterranean.  There  was  discovered  in 
them  a  dust  of  marshy  and  ferruginous  clay,  mixed  up  with  fine  sand, 
which,  as  it  passed  through  the  atmosphere,  deprived  it  of  a  portion  of 
the  organic  matters  in  suspension  there.  In  this  way  these  rams  serve 
a  fertilizing  purpose,  being  in  fact  showers  of  manure.  Each  heavy  gust 
of  wind  raises  clouds  of  dust,  as  may  especially  be  remarked  when,  ani- 
mated by  a  gyratory  movement,  it  possesses  a  certain  force  of  aspira- 
tion which  enables  it  to  form  those  small  whirlwinds  of  dust  which 
may  be  seen  upon  the  high-roads. 


408  THE  ATMOSPHERE. 

The  whole  extent  of  the  vast  zone  of  deserts  which  reaches  over  the 
intertropical  and  the  subtropical  countries  of  the  Old  as  of  the  New 
World  contains  ea'rthy  elements,  which  the  wind  drives  to  an  immense 
distance.  Europe,  like  Asia,  Africa,  and  America,  furnishes  the  wind 
with  a  supply  of  this  kind. 

We  have  already  pointed  out  the  powers  of  whirlwinds.  To  cite 
but  that  of  1780:  it  developed  its  force  near  Carcassonne,  upon  the 
banks  of  the  Aude,  raised  high  into  the  air  immense  quantities  of  sand, 
unroofed  eighty  houses,  and  blew  in  all  directions  stacks  of  wheat 
standing  in  fields.  Large  ash-trees  were  uprooted,  and  their  biggest 
branches  carried  to  a  distance  of  forty  yards.  Such  a  power  amply  ex- 
plains the  fact  of  earth  and  sand  being  taken  so  much  farther.  The 
shower  of  blood  which  fell  at  Sienna  on  December  28-31, 1860,  ana- 
lyzed by  D.  Campani,  seemed  to  be  of  organic  origin. 

One  of  the  latest  showers  of  blood  recorded  is  that  which  occurred 
on  March  10, 1869.  .  On  this  day  the  sirocco  was  blowing  at  Naples, 
and  its  squalls  were  accompanied  by  that  nebulosity  which  is  peculiar 
to  it,  and  which  resembles  a  slight  mist ;  the  barometer  had  fallen  con- 
siderably ;  the  weather  was  very  warm,  and  from  time  to  time  there 
fell  sharp  but  short  showers,  either  of  very  fine  rain  or  in  large  drops  ; 
each  drop  of  this  rain  left  a  muddy  spot  behind  it. 

These  spots,  when  examined  carefully,  had  a  marked  yellowish  brown 
tint,  and  resembled  spots  left  by  water  containing  iron.  A  sheet  of 
white  paper,  first  damped  and  then  exposed  to  the  wind,  was  soon  cov- 
ered with  a  number  of  small  arid  reddish  grains,  nearly  spherical  in 
shape,  the  diameter  of  which  varied  from  0'004  inch  to  0'0004  inch. 
There  can  be  no  doubt,  considering  the  direction  of  the  wind  at  the 
time,  that  these  grains  of  sand  came  direct  from  the  desert  of  Sahara. 

M.  Breton,  of  Grenoble,  noticed  that  this  residue  was  exactly  analo- 
gous to  that  which  was  picked  up  at  Valence  in  September,  1846,  after 
the  red  rain  spoken  of  above.  As  was  imagined,  this  sand  came  from. 
Sahara.  It  appears  from  another  account  that  Algeria  was  the  theatre 
of  a  very  violent  hurricane  on  March  3, 1869. 

French  soldiers  were  overtaken  by  the  wind,  near  El-Outaia,  in  the 
midst  of  a  sea  of  sand.  It  took  them  four  hours  to  travel  six  and  three- 
quarter  miles.  "  During  the  seventeen  years  that  I  have  been  in  Al- 
geria," says  an  eye-witness,  "  I  have  never  seen  such  a  whirlwind.  Our 
little  column  was  compelled  to  stop  and  to  take  precautions  against  be- 
ing killed.  At  the  second  halt  we  turned  our  backs  to  the  squall,  and 


PRODIGIES.  409 

for  an  hour  and  a  half  we  could  see  neither  the  sun  nor  the  sky  al- 
though  just  before  there  had  been  scarcely  any  clouds.  For  more  than 
a  quarter  of  an  hour  together  we  could  not  see  a  distance  of  two  or 
three  yards  in  front  of  us," 

The  red  rain  which  fell  at  Naples  had  undoubtedly  been  brought 
from  the  desert  of  Sahara,  itself  exposed  to  a  tempest  which  in  fact  ex- 
tended over  all  Europe,  the  Mediterranean,  and  Africa. 

These  phenomena  are  intimately  connected  with  the  great  movements 
of  the  atmosphere,  as  M.  Tarry  has  judiciously  pointed  out. 

Ten  days  after  the  red  rain  mentioned  above,  on  the  20th  of  March, 
a  violent  tempest,  coming  from  England,  swept  over  the  north  coast  of 
France.  There  was  a  very  marked  centre  of  atmospheric  depression 
(28.90  inches)  at  Boulogne  on  the  20th ;  by  the  next  day  it  had  reach- 
ed Lesina,  upon  the  Adriatic.  For  several  days  a  violent  north-west 
wind  raged  over  France,  and  afterward  over  Italy.  On  the  22d  the  cy- 
clone had  reached  Africa,  where  it  raised  into  the  air  the  sands  of  Sa- 
hara ;  a  retrograde  movement  then  took  place ;  a  fresh  decrease  of  the 
barometer  reading  occurred  in  the  south  of  Europe,  where  the  pressure 
had  risen  after  the  passage  of  the  cyclone.  On  the  24th  the  barometer 
fell  to  2913  inches  at  Palermo,  and  29-21  inches  at  Eome:  the  wind 
grew  very  violent ;  the  instrument  of  Father  Secchi,  in  the  latter  city, 
indicating  a  speed  of  640  miles  in  the  twenty-four  hours — the  greatest 
of  the  year. 

The  atmosphere  in  Sicily  was  noticed,  on  the  23d,  to  be  laden  with 
thick  clouds  and  a  yellowish  dust,  which  lent  to  the  sky  an  unusual  ap- 
pearance. Rain  falling,  each  .drop  left  a  yellow  residuum,  which  it 
needed  two  or  three  filterings  to  remove.  This  substance,  analyzed  by 
Professor  Silvestre,  at  Catania,  contained  the  following  elements :  clay, 
chalky  sand,  peroxide  of  hydrate  of  iron,  nitrogenized  sodium,  silica, 
and  organic  matter. 

The  same  phenomenon  was  remarked  at  Subiaco,  near  Rome,  and  at 
Lesina,  in  Illyria.  Thus  the  prodigies  spoken  of  by  Livy  are  nov  reg- 
istered at  the  Paris  Observatory. 

The  last  remarkable  red  rain  was  that  of  February  13, 1870.  On 
February  7  a  great  barometrical  depression  occurred  in  England ;  the 
barometer  marked  29.33  inches  at  Penzance ;  on  the  9th  it  had  reached 
the  Mediterranean ;  on  the  10th,  Sicily,  where  the  barometer  reading 
was  lower  than  at  Rome.  This  fall  of  the  barometer  was  accompanied 
by  a  violent  tempest ;  at  Rome  there  was  a  violent  north  wind  for  three 


410  THE  ATMOSPHERE. 

days — the  8th,  the  9th,  and  the  10th.  It  superinduced  a  severe  frost  in 
France  and  Italy,  snow  falling  in  Rome  on  the  nights  of  the  8th  and  the 
9th.  On  the  llth  and  12th  the  weather  was  calmer,  and  the  barometer 
reading  increased  again,  the  cyclone  raging  over  the  desert  of  Sahara. 
The  retrograde  movement  alluded  to  above  soon  made  itself  manifest. 
On  the  12th  the  barometer  fell  to  2945  inches  in  the  south  of  Spain;  a 
violent  wind  from  the  south  blew  over  Spain  and  Italy  on  the  13th  and 
14th ;  and  from  Africa  the  cyclone,  accompanied  by  the  hurricane,  again 
made  its  way  back  to  Europe,  with  the  sand  swept  up  from  Sahara.  As 
a  matter  of  fact,  at  2  P.M.  on  the  13th  of  February,  a  reddish  sand  was 
remarked  in  the  rain  that  fell  at  Subiaco,  near  Rome,  by  M.Alvarez; 
at  Tivoli,  by  Father  Ciampri ;  and  at  Mondragone,  by  Father  Lavaggi. 
In  the  night  of  the  13th  to  the  14th  there  fell  at  Genoa  an  earthy  and 
reddish  substance;  and  at  Moncalieri,  Father  Denza,  Director  of  the 
Observatory,  picked  up  some  red  snoiv  which  contained  the  same  kind 
of  sand. 

This  recital  of  the  showers  of  blood  shows  us — 1st,  that  they  are  a 
reality ;  2d,  that  they  are  mostly  due  to  dust  taken  up  by  the  wind  into 
very  distant  regions ;  3d,  that  they  are  not  so  infrequent  as  they  appear 
to  be.  Thus  there  are  no  less  than  twenty-one  occasions  upon  which 
they  have  been  known  to  occur  during  the  present  century  in  Europe 
and  Algeria,  as  the  following  table  will  show : 

1803.  February Italy. 

1813.  February Calabria. 

1814.  October Oneglia,  between  Nice  and  Genoa. 

1819.  September Studein,  Moravia. 

1821.  May Giessen. 

1839.  April ;...Philippeville,  Algeria. 

1841.  February Genoa,  Parma,  Cariigon. 

1842.  March Greece. 

1846.  May Syam,  Chambery. 

1846.  October .• Dauphine,  Savoy,  Vivarais. 

1847.  March Chambery. 

1852.  March Lyons. 

1854.  May Horbourg,  near  Colmar. 

1860.  31st  December Sienna. 

1862.  March Beaunan,  near  Lyons. 

1863.  March Rhodes. 

1863.  April Between  Lyons  and  Aragon. 

1868.  26th  April Toulouse. 

1869.  10th  March Naples. 

1869.  23d  March Sicily. 

1870.  13th  February Rome. 


PRODIGIES. 


411 

It  will  be  noticed  that  these  remarkable  showers  mostly  take  place 
in  the  spring  and  the  autumn,  at  the  epoch  of  the  equinoctial  gales. 
We  have  seen  that  they  may  be  due  to  the  traces  left  by  certain  kinds 
of  butterfly.  A  third  cause  must  also  be  noticed — viz.,  volcanoes,  the 
ashes  of  which  are  sometimes  conveyed  by  the  winds  to  an  immense 
distance.  Several  cases  in  proof  of  this  might  be  adduced. 

We  now  come  to  another  series  of  remarkable  showers  spoken  of  in 
ancient  legends,  exaggerated  and  interpreted  in  different  ways,  and  the 
true  explanations  of  which  it  is  not  always  easy  to  give. 

Showers  of  milk  are  often  spoken  of  as  having  taken  place.  Thus 
Obsequens  relates  that  upon  the  territory  of  Yeies  there  fell  a  shower 
of  milk  and  oil  in  629.  The  absence  of  all  definite  information  upon 
facts  of  this  kind  prevents  one  from  doing  more  than  hazard  a  few  con- 
jectures borrowed  from  volcanic  eruptions  or  the  carrying  into  the  air 
of  white  or  chalky  earth  by  some  hurricane.  In  620  streams  of  milk 
are  said  to  have  flowed  into  the  Koman  lake.  In  643  milk  is  reported 
to  have  flowed  for  three  days  in  some  place  not  mentioned;  numerous 
victims  were  immolated  when  this  prodigy  took  place.  These  so-called 
streams  of  milk  are  'a  common  phenomenon  in  some  countries;  the 
washing  of  the  rain  over  a  white  soil  suffices  to  cause  this  illusion, 
which,  however,  the  most  cursory  analysis  would  dispel. 

Dion  Cassius  speaks  of  a  rain  that  looked  like  milk,  and  which,  fall- 
ing on  coins  or  copper  vessels,  made-  them  retain  the  appearance  of  sil- 
ver for  three  days.  If  this  fact  be  true,  it  is  clear  that  it  must  have 
arisen  from  a  downfall  of  sublimated  mercury  which  had  become  con- 
densed, and  consequently  had  fallen  to  the  ground.  But  in  what  way 
this  sublimation  and  condensation  was  brought  about,  it  is  first  neces- 
sary to  ascertain,  before  believing  in  the  occurrence  of  this  prodigy. 

Glycas'  also  speaks  of  a  shower  of  mercury,  which  might  be  the  same 
as  the  above,  though  it  is  stated  to  have  taken  place  during  the  reign  of 
Aurelian. 

We  may  compare  with  these  showers  a  phenomenon  which  has  been 
observed  too. often  to  permit  of  its  reality  being  questioned.  I  allude 
to  the  appearance  of  crosses  upon  men's  clothes,  a  few  instances  of  which 
I  append : 

In  764  the  misbehavior  of  the  monks  of  St.  Martin  drew  down  the 
anger  of  God.  Blood  fell  from  the  heavens  on  to  the  earth,  and  crosses 
appeared  upon  men's  garments.—  Gregory  of  Tours. 

Fritsch  speaks  of  the  same  phenomenon  as  occurring  in  783.     In 


412-  THE  A  TM 0  SPHERE. 

1094  crosses  fell  from  heaven  on  to  the  garments  of  priests,  for  the  pur- 
pose, no  doubt,  of  warning  them  of  their  impiety,  says  G.  Schott.  In 
1534  there  fell  in  Sweden  a  shower  which  left  the  mark  of  a  red  cross 
upon  men's  garments.  Cardan  explains  this  phenomenon  by  the  state- 
ment that  red  dust  was  diluted  in  the  rain-water,  and  that  the  crosses 
were  formed  by  the  drops  falling  in  the.  woof  of  the  cloth.  Fromond 
and  Schott  do  not  accept  this  explanation,  because,  according  to  them, 
these  crosses  were  formed  not  only  upon  certain  parts  of  the  garment, 
but  all  over  it,  and  that  when  drops  of  blood  fall  upon  a  piece  of  cloth 
they  never  take  this  shape.  The  pious  of  that  date  considered  it  to  be 
a  direct  intervention  of  the  Deity.  But  this  is  not  all.  It  is  related 
that  in  1501  crosses  fell  in  Germany  and  Belgium,  not  only  upon  the 
garments,  even  when  inclosed  in  boxes,  and  especially  upon  the  garments 
of  women,  but  that  they  left  a  mark  upon  the  skin,  and  upon  bread. 
This  prodigy  lasted  three  years,  recurring  during  Passion-week  and 
Easter ;  no  doubt,  adds  the  chronicler,  to  inspire  the  respect  too  often 
forgotten  to  the  blood  and  cross  of  the  Lord. 

John  of  Horn,  Prince  of  Liege,  told  the  Emperor  Maximilian  I.  of  a 
young  woman  of  that  town,  twenty-two  years  of  age,  whose  garments 
were  perpetually  covered  with  blood-red  crosses,  although  she  continu- 
ally changed  her  clothes. 

It  must,  at  the  same  time,  be  mentioned  that  many  instances  are  cited  * 
in  which  nutritious  substances  have  descended  in  a  shower.  Thus  in 
1824  and  1828  there  was  so  abundant  a  shower  of  this  kind  in  one  of 
the  districts  of  Persia  that  it  covered  the  ground  to  the  depth  of  five  or 
six  inches.  It  was  a  kind  of  lichen,  of  a  sort  already  known ;  cattle 
and  sheep  devoured  it  greedily,  and  some  bread  was  even  made  from  it. 

We  may  also  class  with  the  preceding  the  descent  of  a  soft  substance 
which  Muschenbroeck  states  to  have  occurred  in  Ireland  in  1675.  This 
was  a  glutinous  and  fat  substance,  which  softened  when  held  in  the 
hand,  and  emitted  an  unpleasant  smell  when  exposed  to  the  action  of 
fire. 

On  the  10th  of  March,  1695,  at  about  7  P.M.,  a  heavy  storm  burst 
over  Chatillon-sur-Seine :  the  front  part  of  the  cloud  appeared  inflamed, 
the  air  to  be  on  fire,  and  the  spectators  who  saw  it  believed  that  the 
neighboring  villages  were  being  burned,  as  sparks  of  flame  fell  to  the 
ground  in  all  directions.  This  shower  lasted  a  quarter  of  an  hour,  and 
extended  over  a  large  tract  of  country,  where  it  caused  no  conflagration ; 
immediately  after  the  storm  there  was  a  heavy  fall  of  large  snow-flakes. 


PRODIGIES. 


413 


In  828  there  fell  from  the  sky  a  number  of  grains  like  those  of  wheat, 
but  much  smaller. 

This  fact  may  easily  be  credited,  as  also  the  following,  which  is  told 
by  Johnston :  There  fell  for  the  space  of  two  hours,  over  a  tract  of 
country  two  miles  in  extent,  in  Carinthia,  a  shower  of  wheat  with  which 
bread  was  afterward  made. 

We  may  also  accept  the  statement  'of  Cassiodorus,  that  there  fell  in 
371  a  shower  of  rain,  in  the  country  of  the  Atrebates,  in  which  there 
was  a  plentiful  admixture  of  wool. 

The  showers  of  sulphur,  which  are  often  spoken  of,  are,  as  a  rule, 
nothing  more  than  the  pollen  of  certain  plants,  pine  and  nut  trees  in 
particular,  which  may  be  carried  by  the  wind  to  an  immense  distance. 
Without  going  so  far  back  as  the  storm  of  sulphur  which  destroyed 
Sodom  and  Gomorrah,  there  are  certain  storms  of  the  kind  which  ap- 
pear well  authenticated.  Olaus  Wormius  states  that  on  May  16, 1646, 
there  fell  a  -heavy  shower  at  Copenhagen  which  inundated  the  whole 
city,  and  contained  a  dust  exactly  like  sulphur,  both  in  regard  to  color 
and  smell.  Simon  Paulli  states  that  on  May  19,  1665,  there  raged  in 
Norway  a  fearful  tempest,  with  a  dust  so  like  sulphur  that,  when  thrown 
into  the  fire,  it  produced  the  same  smell,  and  that,  when  mixed  with 
spirits  of  turpentine,  it  produced  a  liquor  the  odor  of  which  was  just  like 
that  of  balm  of  sulphur.  The  close  neighborhood  of  the  Iceland  vol- 
canoes is  sufficient  to  explain  this  occurrence.  Phenomena  of  the  same 
kind  are  not  infrequent  in  Naples.  Sigesbek,  in  the  "  Breslau  Memoirs," 
speaks  of  a  shower  of  sulphur  which  fell  in  Brunswick,  and  which  was 
a  regular  mineral  sulphur.  This  fact  can  not  be  accepted  without  further 
proof:  as  to  the  showers  of  pollen,  flowers,  leaves,  etc.,  they  are  well 
authenticated. 

At  Autreche  (Indre  et  Loire),  at  1210  P.M.  on  April  9,  1869,  the  air 
was  very  still,  and  the  sky  cloudless.  M.  Jallois  relates  that  one  of  his 
correspondents  remarked  a  shower  of  dry  oak-leaves  falling  from  the 
higher  regions  of  the  atmosphere.  Being  gifted  with  excellent  sight, 
he  saw  them  appear  like  bright  specks  upon  the  azure  of  the  sky,  at  a 
very  great  height,  and  fall  about  him,  after  having  descended  almost 
vertically,  with  a  trifling  inclination  eastward.  This  continued  for  ten 
minutes ;  but  the  shower  of  leaves  had  probably  commenced  previous- 
ly. There  was  at  least  one  to  each  square  yard  upon  a  piece  of  water 

close  by. 

This  phenomenon  seems  to  have  resulted  from  a  great  squall  which 


414  THE  ATMOSPHERE. 

occurred  on  April  3 ;  the  oak  leaves  carried  up  by  a  hurricane  into  the 
higher  regions  of  the  atmosphere  were  kept  there  by  the  wind  for  six 
days,  and  fell  again  when  the  weather  became  calm. 

This  shower  of  leaves  reminds  me  of  a  shower  of  oranges.  On  July 
8,  1833,  a  water-spout,  which  took  place  at  Pausilippus,  near  Naples, 
burst  upon  the  shore  and  swept  off  two  large  baskets  of  oranges;  a 
few  minutes  afterward  they  descended  to  the  ground  at  some  distance. 

After  the  vegetable  showers  we  come  to  a  series  even  more  remark- 
able, and  perfectly  well  authenticated.  I  refer  to  the  showers  of  live 
animals. 

In  the  chapter  on  water-spouts  we  have  seen  that  fish  are  sometimes 
taken  up  in  this  way  out  of  a  pond.  Peltier  relates  that  frogs  once  fell 
upon  his  head  from  a  water-spout.  This  was  at  Ham,  in  1835,  and  the 
fact  was  duly  certified.  I  may  cite  another,  still  more  recent. 

In  the  morning  of  January  30, 1869,  toward  4'30  A.M.,  after  a  vio- 
lent gust  of  wind,  there  began  a  fall  of  snow  which  lasted  until  day- 
light, at  Arache,  in  Upper  Savoy;  and  in  the  morning  a  large  number 
of  live  larvae  were  found  in  the  snow.  They  could  not  have  been 
hatched  in  the  neighborhood,  for,  during  the  days  preceding,  the  tem- 
perature had  been  very  low.  On  January  24  the  thermometer  had 
marked  60*8°,  and  upon  the  following  days  a  temperature  of  41°  at 
7  A.M.  They  seemed  to  be  mostly  the  Trogosita  mauritanica,  which  is 
common  in  the  forests  of  Southern  France.  There  were  also  found 
among  them  a  few  caterpillars  of  a  small  butterfly  belonging  to  the 
noctmlian  tribe,  probably  the  Stibia  stagnicola.  This  caterpillar  reaches 
its  full  size  in  the  course  of  February,  and  is  indigenous  to  the  centre 
and  the  south  of  France. 

This  shower  of  insects  at  Arache,  at  an  altitude  of  from  1000  to 
1200  yards,  can  only  be  explained  by  a  violent  wind  which  must  have 
brought  them  from  some  locality  in  the  south  of  France. 

M.  Tissot,  the  village  school-master;  who  observed  this  phenomenon, 
adds,  that  in  the  course  of  November,  1854,  the  wind  being  very  vio- 
lent, thousands  of  insects,  most  of  them  alive,  alighted  upon  a  planta- 
tion near  Turin ;  some  of  them  were  larvae,  and  others  had  attained 
their  full  growth,  while  all  belonged  to  an  order  of  hemiptera  which 
are  nowhere  seen  except  in  the  island  of  Sardinia.  Ancient  authors 
have  related  several  instances  of  falls  of  insects. 

Phanias,  cited  by  Porta,  states  that  there  fell  a  shower  of  fish  for 
three  days  in  the  Chersonesus. 


PRODIGIES.  415 

In  Athens,  Philarcus  asserts  that  he  saw  large  quantities  of  fish  and 
frogs  fall  from  the  sky  in  many  different  places.  Heraclides  Lembus, 
in  Book  XXI.  of  his  "  Histories,"  says  that  God  sent  showers  of  frogs 
upon  Poenia  and  Dardania  in  such  large  quantities  that  the  houses  and 
roads  were  covered  with  them.  They  were  found  mixed  up  in  the 
food,  and  were  consumed  with  it.  The  water  was  filled  with  them ;  it 
was  impossible  to  walk  without  treading  upon  them.  The  decomposi- 
tion of  their  bodies  produced  such  an  odor  that  it  was  found  necessary 
to  quit  the  country. 

Yarro  declares  that  all  the  inhabitants  of  a  certain  town  in  Gaul 
were  driven  from  their  houses  on  account  of  the  countless  frogs  which 
fell  from  the  sky. 

Scaliger  states  that  the  town  of  Mirabel,  in  Aquitania,  was  filled  with 
half-formed  frogs  which  fell  from  the  sky.  Johnston  relates  that,  in 
the  island  of  Auckland  (Friesland),  "in  which  there  are  no  frogs,"  a 
number  fell  in  a  shower  of  rain.  Olaus  Magnus  also  states  that  frogs, 
worms,  and  fish  fall  from  the  clouds  in  the  north  oftener  than  in  the 
south,  "on  account  of  the  viscosity  of  the  clouds  and  the  heat  which 
they  derive  from  the  sulphurous  principle !." 

Fromond  relates  that,  while  standing  with  several  friends  at  one  of 
the  gates  of  Tournai,  in  1625,  a  shower  of  rain  suddenly  fell,  and  pro- 
duced so  many  frogs,  all  of  the  same  size  and  color,  that  the  ground 
was  covered  with  them. 

Porta  says  that  he  often  saw,  between  Naples  and  Pouzzoles,  a  quan- 
tity of  frogs  suddenly  emerge  from  the  dust  upon  which  a  heavy  show- 
er of  rain  had  just  fallen.  This  peculiarity,  he  adds,  is  well  known  to 
many  inhabitants  of  these  two  towns. 

These  sudden  appearances  of  frogs  and  toads  are  generally  due  to 
the  fact  that  these  animals  mostly  issue  from  the  mud  after  a  thunder- 
storm, and  are  in  the  habit  of  crossing  frequented  routes.  It  is  excess- 
ively rare  for  whirlwinds  to  carry  up  into  the  air  either  fishes  or  frogs. 
The  showers  of  locusts  are  due  to  flying  masses  of  orthoptera,  the 
nomad  cricket  in  particular.  These  insects  are  a  scourge  to  agricul- 
ture. They  are  brought  by  the  wind ;  and  when  they  alight,  they 
transform  a  fertile  region  into  a  desert.  Seen  from  a  distance,  their 
countless  swarms  present  the  appearance  of  thunder -clouds.  These 
dark  masses  hide  the  sun.  As  far  and  as  high  as  the  eye  can  reach, 
the  sky  is  black  and  the  ground  covered  with  them.  The  sound  of 
their  million  wings  is  like  the  noise  of  a  cataract  As  they  reach  the 


416  THE  ATMOSPHERE. 

ground,  they  break  the  branches  of  the  trees.  In  a  few  hours  all  signs 
of  vegetation  have  disappeared  over  an  extent  of  several  leagues.  The 
wheat  is  gnawed  to  its  roots,  the  trees  are  stripped  of  their  leaves.  Ev- 
ery thing  is  destroyed,  sawn,  cut  to  pieces,  and  devoured.  When  noth- 
ing is  left,  the  terrible  swarm  rises,  as  if  at  a  given  signal,  and  flies  off, 
leaving  famine  and  desolation  behind  it. 

It  often  happens  that,'after  having  consumed  every  thing,  they  die 
of  starvation  before  depositing  their  eggs.  Their  bodies,  heated  by  the 
sun,  soon  become  putrefied,  and  emit  exhalations  which  breed  terrible 
epidemics  in  the  district 

In  1690  locusts  arrived  in  Poland  and  Lithuania  from  three  different 
points,  and  in  three  distinct  masses.  The  Abbe  de  Ussans,  who  saw 
them,  says,  "At  certain  places  where  they  had  died  in  large  quantities 
they  lay  four  feet  deep.  Those  which  were  still  alive,  and  which  had 
settled  upon  the  trees,  made  the  boughs  bend  beneath  their  weight." 

In  1749  locusts  arrested  the  march  of  Charles  XII. 's  army  when  it 
was  retreating  through  Bessarabia  after  the  defeat  of  Pultowa.  The 
king  thought  that  it  was  a  hailstorm  which  was  thus  swooping  down 
upon  his  army.  The  arrival  of  the  locusts  was  announced  by  a  hissing 
sound  like  that  which  precedes  a  tempest,  and  the  rustling  of  their 
flight  drowned  the  sound  of  the  waves  of  the  Black  Sea.  All  the 
country  in  their  track  was  laid  bare. 

In  the  south  of  France  locusts  sometimes  multiply  at  such  a  prodig- 
ious rate  that  they  soon  produce  enough  eggs  to  fill  several  barrels. 
They  have  at  times  caused  terrible  damages ;  notably  so  in  the  years 
1805,  1820,  1822,  1824,  1825,  1832,  and  1834. 

Mezeray  states  that  in  January,  1613,  during  the  reign  of  Louis  XIIL, 
locusts  invaded  the  district  round  Aries.  In  seven  or  eight  hours  all 
the  wheat  and  forage  were  devoured  to  the  very  roots  over  20,000  acres 
of  ground.  They  then  crossed  the  Rhone  and  visited  Tarascon  and 
Beaucaire,  where  they  consumed  the  garden  produce  and  the  lucerne. 
They  went  from  thence  to  Aramon,  Monfrin,  Valebregues,  etc.,  where 
most  of  them  were,  fortunately,  destroyed  by  starlings  and  other  insect- 
eating  birds  which  had  been  attracted  thither  by  the  prospect  of  such 
a  banquet  The  consuls  of  Aries  and  Marseilles  had  their  eggs  picked 
up.  It  cost  the  former  town  25,000  and  the  latter  20,000  francs,  and 
3000  cwt.  of  eggs  were  thrown  into  the  Rhone.  Counting  1,750,000 
eggs  to  the  cwt.,  5,250,000,000  of  locusts,  as  they  would  afterward  have 
become,  must  have  been  destroyed. 


PRODIGIES. 


Fig.  77. — Shower  of  locusts. 


In  1825,  in  the  territory  of  Saintes- Maries,  not  far  from  Aigues- 
Mortes,  upon  the  shores  of  the  Mediterranean,  1518  wheat-sacks  were 
filled  with  dead  locusts,  the  weight  of  which  was  nearly  sixty-nine 
tons;  at  Aries  there  were  picked  up  165  sackfuls,  or  between  six  and 
seven  tons. 

Locusts  are  always  to  be  met  with  in  Algeria,  in  the  provinces  of 
Oran,  Bone,  Algiers,  and  Bougie;  but  they  are  not  so  numerous  as  to 
produce  those  terrible  invasions  which  change  a  fertile  country  into  a 
desert  There  are  locust  years  in  Algeria,  just  as  in  France  there  are 
years  when  beetles,  caterpillars,  etc.,  are  especially  abundant.  These 
scourges  are,  fortunately,  very  rare.  The  most  disastrous  took  place  in 
1845  and  1866. 

Eegular  showers  of  beetles  have  also  been  known  to  descend  like  a 
thick  cloud  and  cover  the  fields  and  the  highways. 

27 


418 


THE  ATMOSPHERE. 


As  with  the  locusts,  they  swarm  from  one  province  into  another. 
Masses  of  these  coleoptera,  which  are  not  transported  by  a  whirlwind, 
but  which  are  generally  driven  by  the  wind,  emigrate  from  a  district 
after  they  have  devoured  every  thing  in  it. 

To  give  an  idea  of  the  prodigious  numbers  in  which  cock-chafers 
sometimes  make  their  appearance,  I  will  quote  some  few  historical 
instances. 

In  157-i  these  insects  so  abounded  in  England  that  they  stopped 
several  mills  on  the  Severn. 

In  1688  they  formed  so  dense  a  cloud  in  Galway  that  the  sky  was 
darkened  to  the  distance  of  a  league,  and  the  peasants  had  a  difficulty 
in  finding  their  way  about.  They  destroyed  all  vegetation,  so  that  the 
country  around  had  the  look  of  winter.  Their  voracious  jaws  made  a 
noise  like  that  caused  by  the  sawing  of  a  thick  piece  of  timber;  and 


:' 


78.—  Shower  of  cock-chafers. 


PRODIGIES. 


in  the  evening  the  flapping  of  their  wings  resembled  the  distant  rolling 
of  a  drum.  The  unhappy  Irish  were  compelled  to  cook  and  eat  them 
for  want  of  other  food.  In  1804  vast  clouds  of  cock-chafers,  precipi- 
tated by  a  violent  wind  into  the  Lake  of  Zurich,  formed  a  thick  mass 
upon  the  shore,  where  their  bodies  were  heaped  up,  the  putrid  exhala- 
tions from  which  poisoned  the  atmosphere.  On  May  18, 1832,  at  9  P.M., 
a  legion  of  beetles  encountered  a  diligence  upon  the  route  from  Gour- 
nay  to  Gisors  (as  it  was  leaving  Talmoutiers)  with  so  much  violence 
that  the  horses,  blinded  and  frightened,  were  compelled  to  return. 

Such  is  the  series  of  showers  of  blood,  earth,  vegetables,  and  animals, 
which  the  history  of  meteorology  has  registered.  We  will  stop  here. 
Just  as  in  the  preceding  chapter  we  saw  that  there  were  writers  who 
spoke  of  hailstones  as  big  as  elephants,  so  too,  in  this  case,  there  has 
been  considerable  exaggeration.  Fabulous  as  may  be  the  force  which 
the  wind  sometimes  acquires,  we  may  assign  to  the  domain  of  romance 
the  story  told  by  Avicenne,  that  prince  of  Arab  doctors,  as  to  his  hav- 
ing seen  the  body  of  a  calf  fall  from  the  skies.  Nevertheless,  Xavier 
de  Maistre  declares  that  a  young  girl  was  carried  off  by  a  whirlwind  in 
1820 ;  but  it  is  not  said  to  what  height.  Cabeus,  in  the  seventeenth 
century,  declared  that  a  violent  wind  had  blown  away  a  woman  who 
was  washing  linen  in  the  lake.  In  regard  to  large  animals,  the  most 
exaggerated  story  is  the  one  which  is  also  the  oldest — viz.,  as  to  the 
Nemasan  Lion  falling  from  the  moon  on  to  the  Peloponnesus.  ...  It 
is  true  that  stones  to  the  weight  of  hundreds  of  pounds  sometimes  fall 
from  the  sky,  as  we  saw  in  regard  to  aerolites.  But  hitherto  the  other 
worlds  have  sent  us  nothing  more  valuable  than  stones.  The  animals, 
fish,  insects,  grains,  and  leaves,  which  fall  from  the  sky  come  originally 
from  the  earth,  not  from  any  of  the  planets. 


BOOK  SIXTH. 

ELECTRICITY,  THUNDER-STORMS,  AND  LIGHTNING. 


ELECTRICITY  UPON  THE  EARTH. 


CHAPTEE  I. 

ELECTRICITY  UPON  THE  EARTH  AND  IN  THE  ATMOSPHERE  :  ELECTRIC 
CONDITION  OF  THE  TERRESTRIAL  GLOBE  —  DISCOVERY  OF  ATMOS- 
PHERIC ELECTRICITY— EXPERIMENTS  OF  OTTO  DE  GUERICKE,  WALL, 
NOLLET,  FRANKLIN,  ROMAS,  RICHMANN,  SAUSSURE,  ETC.— ELECTRICITY 
OF  THE  SOIL,  OF  THE  CLOUDS,  OF  THE  AIR— FORMATION  OF  THUN- 
DER-STORMS, 

WE  now  come  to  the  most  marvelous  and  singular  agent  that  exists, 
the  study  of  which  will  complete  and  close  the  immense  panorama  de- 
veloped in  this  work— viz.,  electricity,  thunder-storms,  and  lightning. 
The  study  of  them  is  by  no  means  devoid  of  complications ;  but  our 
close  attention  will  be  amply  repaid  by  the  wonderful  spectacles  which 
it  will  reveal.  Following  our  general  plan,  we  will  see  how  it  is  dis- 
tributed over  the  earth  and  in  the  atmosphere.  It  is,  however,  first 
necessary  to  obtain  an  idea  of  its  history,  which  is  somewhat  remark- 
able. 

Otto  de  Guericke,  burgomaster  of  Magdeburg,  the  celebrated  inventor 
of  the  pneumatic  machine,  first  discovered  (about  1650)  some  signs  of 
electric  light.  Dr.  Wall,  at  about  the  same  epoch,  by  applying  friction 
along  a  cylinder  of  amber,  saw  a  bright  spark  emitted,  and  heard  a 
sharp  noise ;  and,  curiously  enough,  this  first  electric  spark  produced 
by  the  hand  of  man  was  at  once  compared  to  the  lightning's  flash. 
This  light  and  this  sound,  says  Dr.  Wall,  in  his  "Memoirs"  (see  "Phil. 
Trans."),  seem  to  represent,  in  a  certain  measure,  the  lightning  and  the 
thunder.  The  analogy  was  striking,  and  needed  only  an  effort  of  the 
imagination  to  be  understood ;  but  to  demonstrate  its  truth,  to  discover 
in  so  insignificant  a  phenomenon  the  causes  and  the  laws  of  the  great- 
est phenomena  in  nature,  required  a  series  of  proofs  which  could  only 
be  expected  from  a  great  genius.  Nevertheless,  many  physical  philos- 
ophers endeavored  to  obtain  them  by  comparisons  of  a  more  or  less  in- 
genious kind:  some  remarked  that  the  spark  is  zigzag,  like  lightning: 
others  opined  that  thunder  in  the  hands  of  nature  is  the  same  as  elec- 
tricity in  the  hands  of  man.  "  I  confess,"  said  Abbe"  Nollet,  "  that  I 
should  look  upon  this  idea  with  great  complacency  if  it  could  be  well 


424  THE  ATMOSPHERE. 

sustained ;  and  thers  are  many  specious  reasons  by  which  it  might  be." 
Still,  this  was  nothing  more  than  a  train  of  reasoning  which  could  not 
be  conclusive,  inasmuch  as  in  physics  experiment  alone  is  absolutely 
decisive.  While  Europe  and  the  whole  of  the  Old  World  were  thus 
reasoning,  America  was  conducting  experiments  in  special  reference  to 
the  subject  of  lightning.  Franklin  succeeded  in  bringing  electricity 
down  from  the  sky,  in  order  to  investigate  it  by  direct  examination. 
After  having  made  several  discoveries  in  respect  to  electricity,  especial- 
ly in  regard  to  the  Leyden  jar  and  the  attractive  power  of  fine  points, 
Franklin  went  in  search  of  electricity  into  the  very  midst  of  the  clouds. 
He  had  concluded,  as  the  result  of  certain  experiments,  that  a  stem  of 
pointed  metal,  placed  at  a  great  height,  upon  the  summit  of  a  building, 


Fig.  79.— Experiments  of  Fraiiklin  and  Romas. 

formed  a  receptacle  for  the  electricity  of  thunder-clouds.  He  was  await- 
ing, with  no  little  impatience,  the  construction  of  a  steeple  then  being 
built  at  Philadelphia ;  but  unwilling  to  remain  so  long  in  doubt,  he 
had  recourse  to  a  more  expeditious  and  not  less  certain  method  for  as- 
certaining what  he  desired  to  know.  As  all  that  was  necessary  was  to 
raise  a  substance  of  some  kind  into  the  region  of  the  thunder — that  is 
to  say,  high  enough  into  the  air — he  thought  that  an  ordinary  kite 
would  'serve  his  purpose  as  well  as  any  steeple.  He  accordingly  ar- 
ranged two  pieces  of  stick,  laid  crosswise  and  covered  with  a  silk  hand- 
kerchief, which  he  took  into  the  fields  upon  the  occasion  of  the  first 
thunder-storm.  Fearing  the  ridicule  which  failure  would  entail,  he  was 
accompanied  only  by  his  son.  The  kite  remained  some  time  in  the  air 


ELECTRICITY  UPON  THE  EARTH.  425 

without  any  perceptible  effect  being  produced ;  but  at  last  the  fibres  of 
the  rope  were  somewhat  agitated.  Encouraged  by  this,  Franklin  placed 
his  finger  upon  the  end  of  the  rope,  a  motion  which  immediately  led  to 
the  appearance  of  a  bright  spark,  which  was  soon  followed  by  several 
others.  Thus,  for  the  first  time,  the  genius  of  man  succeeded  in  play- 
ing with  the  lightning  and  discovering  the  secret  of  its  existence. 

Franklin's  experiment' took  place  in  June,  1752,  and  was  shortly  af- 
terward repeated  in  every  civilized  country  with  the  same  success.  A 
French  magistrate,  De  Eomas,  assessor  to  the  Nerac  Tribunal,  profiting 
by  the  ideas  of  Franklin,  which  had  been  made  public  in  France,  con- 
ceived the  idea  of  using  the  kite  with  raised  bars;  and  in  the  month 
of  June,  1753,  before  the  result  of  Franklin's  experiments  was  known, 
he  had  obtained  very  strong  electric  signs,  because  he  had  prudently 
attached  a  metal  wire  to  the  cord  along  its  whole  length,  which  meas- 
ured 850  feet.  A  little  later,  in  1757,  Eomas  repeated  these  experi- 
ments during  a  thunder-storm ;  and  this  time  he  elicited  sparks  of  an 
enormous  size.  He  says,  "  Imagine  tongues  of  fire  nine  or  ten  feet  in 
length,  and  an  inch  thick,  which  made  as  loud  a  report  as  a  pistol.  In 
less  than  an  hour  I  had  obtained  at  least  thirty  sparks  of  these  dimen- 
sions, to  say  nothing  of  a  thousand  others  of  seven  feet  or  less."  A 
great  number  of  persons,  including  several  ladies,  were  present  at  these 
experiments. 

As  may  be  imagined,  these  experiments  were  not  unattended  with 
danger.  Romas  was  on  one  occasion  knocked  down  by  an  excessively 
heavy  discharge,  but,  fortunately,  escaped  severe  injury.  Richmann,  a 
member  of  the  St.  Petersburg  Academy  of  Sciences,  was  not  so  fortu- 
nate, as  one  of  his  experiments  cost  him  his  life.  He  had  erected  an 
iron  rod,  which  conducted  the  atmospheric  electricity  from  the  roof  of 
the  house  to  his  study,  so  that  he  could  measure  its  intensity  every 
day.  On  the  6th  of  August,  1753,  in  the  midst  of  a  violent  storm, 
and  while  standing  at  some  distance  from  the  rod,  in  order  to  avoid 
the  large  sparks,  he  incautiously  approached  too  near  the  conductor. 
A  globe  of  bluish  fire  struck  him  on  the  forehead,  and  killed  him  on 
the  spot. 

For  the  last  hundred  years  the  study  of  electricity  has  been  pursued 
both  by  experiments  made  in  the  laboratory  and  in  the  atmosphere — 
with  what  splendid  results  it  is  needless  to  relate.  The  electric  tele- 
graph, which  enables  us  to  carry  on  a  whispered  conversation  with  our 
neighbors  across  the  ocean,  and  the  process  which  effects  a  faithful  re- 


426 


THE  ATMOSPHERE. 


Fig.  SO. — Richmann,  of  St.  Petersburg,  struck  by  lightning  during  an  electrical  experiment. 

production  of  the  chefs-d'oeuvre  of  statuary  and  engraving,  are  but  two 
of  the  most  important  applications  of  the  first.  The  experiments  upon 
the  electricity  of  the  atmosphere,  devoted  to  more  complex  and  potent 
phenomena,  have  enabled  us  to  acquire  a  more  exact  notion  concerning 
the  conditions  of  this  electricity  and  its  various  manifestations. 

Electricity  is  a  power  the  inner  nature  of  which,  like  that  of  heat, 
light,  and  attraction,  remains  unknown  to  us.     This  power  produces 


ELECTRICITY  UPON  THE  EARTH.  427 

certain  effects  ;  and  it  is  the  study  of  these  effects  which  constitutes  the 
science.  To  explain  them,  it  is  admitted— first,  that  electricity  is  a  sub- 
tle fluid,  capable  of  becoming  amassed,  condensed,  and  rarefied ;  of  dis- 
charging itself  from  one  body  into  another;  of  traversing  immense  dis- 
tances more  rapidly  even  than  light,  which  itself  travels  at  the  rate 
of  about  185,000  miles  per  second ;  secondly,  that  this  fluid  has  two 
modes  of  existence — two  modes  of  manifesting  itself — which  are  dis- 
tinguished, the  one  from  the  other,  by  the  terms  positive  and  negative. 
These  distinctions  do  not  exist  in  nature,  and  are  only  perceptible  to 
human  sense  by  relative  variations  in  intensity.  Be  this  as  it  may,  it 
has  been  ascertained  that  opposite  electricities  attract,  whereas  like  electrici- 
ties repel  each  other.  The  union  of  equal  quantities  of  fluids  of  an  op- 
posite denomination  forms  a  neutral,  or  natural,  fluid,  which,  it  is  be- 
lieved, exists  in  inexhaustible  quantities  throughout  all  bodies.  Under 
many  influences,  among  which  must  be  cited  that  of  friction,  the  neu- 
tral fluid  becomes  decomposed  into  one  or  the  other  of  these  two  ele- 
ments. The  terrestrial  globe  and  the  atmosphere  are  two  vast  reser- 
voirs of  electricity,  between  which  there  is  a  constant  exchange  by 
decomposition  and  reconstitution,  which  plays  a  complementary  part 
to  the  action  of  heat  and  moisture  in  the  life  of  plants  and  of  animals. 

The  general  result  of  the  researches  into  the  conditions  of  electricity 
upon  the  surface  of  the  globe  and  in  the  atmosphere  is,  that  in  a  nor- 
mal condition  the  globe  is  charged  with  negative  and  the  atmosphere 
with  positive  electricity.  At  the  surface  of  the  soil,  where  continual  ex- 
changes are  taking  place,  electricity  is  in  a  neutral  state,  as  also  in  the 
lower  stratum  of  air,  which  is  in  contact  with  the  surface,  upon  the  sea 
as  well  as  upon  land.  Positive  electricity  increases  in  the  atmosphere 
in  proportion  to  height. 

The  large  amount  of  evaporation  which  takes  place  from  the  surface 
of  the  sea  in  the  regions  of  the  equator  loads  the  clouds  with  positive 
electricity,  and  these,  carried  by  the  upper  currents,  travel  toward  the 
polar  regions,  and  charge  the  atmosphere  there  with  an  accumulation 
of  this  electricity.  Its  influence  causes  in  the  soil  of  the  polar  regions 
an  opposite  condensation  of  negative  electricity.  The  aurora)  boreales 
are,  in  chief,  caused  by  these  two  conflicting  tensions ;  it  is  a  silent  but 
visible  reconstitution  of  the  natural  fluid  by  the  two  opposite  tensions 
of  the  atmosphere  and  the  soil.  Thus  the  appearance  of  an  aurora  be 
realis  is  accompanied  by  electric  currents,  which  circulate  upon  the  s< 
at  a  distance  sufficiently  great  to  permit  of  the  movements  of  the  mag- 


428  THE  ATMOSPHERE. 

netic  needle,  indicating,  at  the  Paris  Observatory,  for  instance,  an  au- 
rora which  may  be  visible  in  Sweden  or  Norway. 

Clouds  are  generally  charged  with  positive  electricity ;  nevertheless, 
negative  clouds  are  sometimes  met  with.  It  is  not  unusual  to  see  upon 
the  summit  of  a  mountain  clouds  adhering  to  it,  as  if  they  were  attract- 
ed thither,  making  a  halt  there,  and  then  following  the  general  move- 
ments of  the  wind.  It  often  happens  that  in  this  case  the  clouds  lose 
their  positive  electricity  by  their  contact  with  the  mountain,  and  as- 
sume the  negative  electricity  of  the  latter,  which,  far  from  serving  to  at- 
tract, has,  on  the  contrary,  a  tendency  to  repel  and  drive  them  away. 
On  the  other  hand,  a  stratum  of  clouds  situated  between  the  ground, 
negative,  and  an  upper  stratum,  positive,  is  almost  neutral ;  its  positive 
electricity  becomes  accumulated  upon  its  inside  surface,  and  the  first 
drops  of  rain  cause  it  to  disappear  altogether. 

The  electricity  of  the  atmosphere  is  subject,  like  heat  and  atmospheric 
pressure,  to  a  double  annual  and  diurnal  oscillation,  and  to  accidental 
oscillations  greater  than  those  which  are  fixed  and  regular.  The  maxi- 
mum occurs  from  6  to  7  A.M.  in  summer,  and  from  10  A.M.  to  noon  in 
winter;  the  minimum  is  between  5  and  6  P.M.  in  summer,  and  between 
2  and  3  P.M.  in  winter.  A  second  maximum  is  also  noticeable  at  sun- 
set, followed  by  a  diminution  during  the  night  until  sunrise.  This  os- 
cillation is  connected  with  that  of  the  hygrometrical  condition  of  the 
air.  In  the  annual  variation  the  maximum  occurs  in  January  and  the 
minimum  in  July :  it  is  due  to  the  great  atmospheric  circulation.  Win- 
ter is  the  period  when  the  equatorial  currents  are  most  active  in  our 
hemisphere,  and  it  is  then  that  the  aurorae  boreales  are  most  numerous. 

As  the  positive  or  negative  conditions  of  electricity,  as  determined  by 
apparatus  constructed  for  measuring  their  intensities,  are  but  a  compari- 
son more  or  less  between  two  different  charges,  it  follows  that,  when  an 
electric  cloud  passes  over  our  heads  and  dissolves  itself  into  rain,  the  air 
may  manifest  negative  electricity  both  before  and  after  the  rain,  and 
even  during  its  fall,  according  to  the  intensity  of  the  charge  contained 
in  the  cloud.  M.  Quetelet  demonstrates  this  state  of  things  in  the  fol- 
lowing manner : 

Let  A,  B,  C,  D,  E,  be  five  positions  on  the  earth  in  a  straight  line, 
which  we  suppose  to  be  neutral.  The  stratum  of  air  above  and  parallel 
with  the  positions  on  the  earth — A'  B'  C'  D'  E' — is  in  a  state  of  positive 
electricity  in  the  absence  of  clouds,  and  to  an  equal  extent  throughout. 

The  stratum  A"  B"  C"  D"  E",  still  higher  and  parallel,  is  also  in  a 


ELECTRICITY  UPON  THE  EARTH.  429 

state  of  positive  and  more  intense  electricity.  There  comes  suddenly  a 
cloud  at  the  three  central  positions  B'  C'  D',  in  a  state  of  positive  elec- 
tricity, greater  than  that  of  the  circumambient  air.  It  follows  that,  rel- 
atively to  it,  the  air  which  is  around  will  display  negative  electricity. 

To  an  observer  situated  at  A,  the  electricity  above  the  earth  will'be 
positive.  As  the  cloud  approaches,  these  indications  will  become  gradu- 
ally less  until  they  vanish  altogether,  and  even  become  negative  on  the 
passage  of  the  cloud.  But  the  rain  will  bring  back  positive  electricity. 
A  corresponding  variation  will  be  manifest  when  the  rain  stops,  and  the 
cloud  moves  off.  At  D,  the  indications  will  be  negative ;  at  E,  they 
will  again  become  positive. 

We  saw  in  the  conflicts  of  the  great  atmospheric  currents  in  the  trop- 
ical regions,  where  the  node  of  the  circuit  accomplished  from  the  equator 
to  the  poles  takes  place,  that  the  evaporation  of  the  seas,  caused  by 
solar  heat  in  these  foci  of  condensation,  the  variation  of  atmospheric 
pressure,  etc.,  engender  cyclones,  hurricanes,  and  tempests,  the  whirling 
march  of  which  reaches  as  far  as  our  temperate  latitudes.  These  vio- 
lent movements  develop  electricity  in  immense  proportions,  and  it  is 
rarely  that  these  phenomena  are  not  accompanied  by  thunder-storms, 
lightning,  and  thunder.  The  formation*  of  the  clouds  upon  sea  and 
land,  the  fogs  which  occur  in  our  regions,  the  course  of  the  clouds  along 
our  valleys  and  mountains,  all  emit  varying  quantities  of  electricity. 
There  is  a  storm  when  this  electricity  of  the  clouds,  instead  of  effecting 
a  mutual  and  tranquil  interchange,  collects  at  certain  points,  and,  becom- 
ing condensed,  saturates  them,  so  to  speak,  and  finally  bursts,  afterward 
uniting  itself  to  the  negative  electricity  which  has  been  instantaneously 
amassed,  either  upon  the  ground  or  in  other  clouds. 

The  great  storms  reach  us  from  the  Atlantic.  They  arise  from  the 
cyclones,  and  the  clouds  which  convey  them  are  generally  more  than 
3000  or  5000  feet  high,  traveling  from  S.W.  to  N.E.,  without  being  ap- 
parently affected  by  the  undulations  of  the  ground  in  France.  The  sec- 
ondary storms,  which  are  formed  in  France  itself,  are  conveyed  by 
clouds  of  a  less  elevation  than  the  above,  and  which  sometimes  just 
skim  the  ground,  being,  in  fact,  influenced  by  it,  scarcely  reaching  over 
the  mountains,  following  the  valleys,  amidst  which  they  distribute  in 
large  quantities  lightning  and  hailstorms. 

The  formation  of  storms  is  preceded  by  a  slow  but  steady  decline  in 
the  reading  of  the  barometer.  The  -calm  of  the  air  and  a  stifling  heat, 
due  to  the  absence  of  evaporation  from  the  surface  of  our  bodies,  are 


430  THE  ATMOSPHERE. 

specially  characteristic  circumstances.  The  variations  in  the  electric 
condition  of  the  soil  and  the  atmosphere,  added  to  the  above,  have  a 
great  effect  upon  our  organic  system.  A  peculiar  nervous  feeling,  with 
no  visible  cause  for  it,  takes  possession  of  many  persons,  in  spite  of  all 
their  efforts  to  shake  it  off.  It  is  under  these  circumstances  that  one  is 
especially  enabled  to  see  how  intimate  is  the  connection  between  man's 
physical  and  moral  condition. 


LIGHTNING  AND  THUNDER.  43  ^ 


CHAPTER  II. 
LIGHTNING  AND  THUNDER. 

WHEN  electricity  is  discharged  from  a  cloud  by  which  it  is  overload- 
ed, and  is  precipitated  either  into  another  cloud  or  to  the  ground  with 
opposite  electricity,  electric  light  is  produced— a  rapid  spark  such  as  we 
display  on  a  small  scale  in  our  experiments  in  physics.  This  spark 
traverses  in  an  instant  the  distance,  whatever  it  may  be,  which  separates 
the  two  electrized  points.  It  .has  been  ascertained  that  it  does  not  last 
10000  of  a  second.  It  is  this  electric  spark  which  constitutes  lightning; 
it  is  by  it  that  lightning  is  made  manifest  during  a  storm. 

As  a  general  rule,  these  flashes  appear  in  the  shape  of  a  sudden  dif- 
fused light  which  illuminates  the  clouds,  the  sky,  and  the  earth,  and  is 
followed  by  a  darkness 'which  seems  more  intense  than  it  was  before  by 
the  force  of  contrast.  Whether  in  this  case  the  exchange  of  electricity 
between  the  clouds  takes  place  simultaneously  over  a  large  surface  which 
is  lighted  up  and  which  dies  away  instantaneously,  or  whether  there  be 
a  spark  as  in  lightning  concealed  by  the  clouds,  in  either  event  one  only 
sees — which  is  of  the  most  frequent  occurrence — a  sudden  diffused  light, 
upon  which  are  momentarily  displayed  the  more  or  less  marked  con- 
tours of  the  clouds. 

These  diffused  lightnings  are  the  most  frequent.  Hundreds  of  flashes 
are  seen  during  a  stormy  day,  or  rather  night,  to  one  flash  of  linear 
lightning.  The  latter  is,  however,  characteristic  lightning.  It  is  but 
a  strong  electric  spark,  a  small  ball  of  fire  which  darts  from  an  over- 
charged cloud  to  the  earth,  or  from  one  cloud  to  another,  or  which  even 
rises  from  the  earth  to  the  clouds ;  the  rapidity  of  its  progress  produces 
the  effect  of  a  narrow  and  luminous  line.  It  is  rare  that  it  darts  in  a 
straight  line,  in  spite  of  the  axiom  as  to  "  the  nearest  road ;"  whether 
because  of  the  varied  distribution  of  moisture  in  the  air,  which  causes  it 
to  be  a  more  or  less  better  conductor,  or  because  of  the  varying  excess 
of  electricity  in  different  parts  of  the  soil  and  of  the  clouds,  the  lightning 
is  nearly  always  zigzag.  The  subtle  fluid  shows,  by  the  way  in  which 
it  traverses  our  dwelling-places,  that  it  leaps  suddenly  from  one  point  to 
another  as  if  by  caprice,  but  being  evidently  obedient  to  the  laws  of  the 


432  THE  ATMOSPHERE. 

distribution  and  conductibility  of  electricity.  Generally  speaking,  linear 
lightning  darts  in  obtuse-angled  zigzags,  or  else  is  curled  like  a  snake. 
Sometimes  it  splits  into  two  or  more  branches.  Nicholson  and  the 
Abb6  Kichard  observed  forked  flashes.  Occasionally,  though  more 
rarely,  it  splits  into  three  branches;  Arago  cites  several  instances  of 
this,  especially  in  the  volcanic  thunder  -  storms ;  Kaemtz  noticed  it 
once.  At  times,  too,  the  flashes  have  four  or  five  ramifications,  or,  it 
may  be,  the  branches  which  issue  from  the  original  flash  become  rami- 
fied into  several  small  lateral  branches.  M.  Liais  observed  and  sketched 
flashes  with  five  branches. 

The  flashes  are  not  always  of  a  shining  white  hue,  but  have  at  times 
a  yellow,  red,  blue,  and  even  a  violet  or  purple  tint;  this  color  depends 
upon  the  quantity  of  electricity  which  traverses  the  air,  upon  the  densi- 
ty of  the  latter,  upon  its  moisture,  and  upon  the  substances  suspended 
in  it.  The  violet  flashes  generally  indicate  that  the  cloud  from  which 
they  are  emitted  is  at  a  great  height,  and  the  air  which  they  travel 
through  an  air  so  rarefied  as  to  call  to  mind  that  of  the  Geissler  tubes. 

It  is  rarely  that  a  correct  idea  as  to  the  length  of  lightning-flashes  is 
formed.  While  we  produce  with  the  greatest  difficulty  in  our  labora- 
tories an  electric  spark  of  a  few  inches,  Nature  shoots  forth  sparks  as 
much  as  ten  miles  long.  F.  Petit  measured  at  Toulouse  some  flashes 
which  were  ten  and  a  half  miles  long — the  extreme  length  with  which 
I  am  acquainted.  Arago  found  that  a  series  of  flashes  which  he  meas- 
ured were  seven  or  eight  miles  in  length. 

In  reply  to  the  question  as  to  the  height  of  thunder-clouds,  it  is  evi- 
dent that  they  are  of  different  elevations.  De  1'Isle  measured  one  on 
June  6, 1712,  which  was  26,250  feet  above  Paris;  Chappe,  on  July  13, 
1761,  remarked  one  that  was  situated  10,400  feet  over  Tobolsk ;  and 
Kaemtz  noticed  another  10,200  feet  above  Halle.  These  observations 
give  a  decreasing  series  of  elevations  which  gradually  decline  until 
they  almost  reach  the  ground.  Haidinger  measured  thunder-clouds 
which  were  only  230  feet  above  Gratz,  on  June  15, 1826,  while  upon 
another  occasion  he  remarked  some  only  ninety-two  feet  above  the 
ground  at  Admont  This  refers  to  a  level  country.  In  the  mountains, 
Saussure  observed  some  of  these  clouds  over  Mont  Blanc;  Bouguer 
and  La  Con'damine,  over  the  Pichincha,  at  16,000  feet;  Eamond,  upon 
Mont  Perdu,  at  .11,100  feet,  and  upon  the  Peak  du  Midi  at  9630  feet, 
and  indeed  at  all  heights.  They  are  generally  from  2950  to  3280  feet 
high  over  the  sea. 


LIGHTNING  AND  THUNDER.  433 

Whether  the  flash  takes  place  horizontally  between  two  groups  of 
clouds,  or  obliquely  either  between  clouds  of  different  strata  or  between 
the  clouds  and  the  ground,  it  is  generally  several  miles  long.  It  is  this 
length  which  is  the  primary  cause  of  the  rolling  of  thunder.  Thunder 
is,  in  reality,  but  the  sound  of  the  electric  spark  effecting  an  exchange 
of  electricity,  a  neutralization,  between  two  points  more  or  less  distant 
from  each  other. 

The  noise  of  the  thunder  may  be  due  to  several  different  causes. 
The  spark  itself,  as  it  traverses  in  an  instant  the  atmospheric  air,  forces 
back  the  molecules  upon  its  passage,  and  produces  a  momentary  void 
into  which  the  circumambient  air  at  once  rushes,  and  so  on  for  a  cer- 
tain distance.  Pouillet  met  this  rather  natural  explanation  by  the  ob- 
jection that  if  the  sound  of  thunder  was  produced  in  this  way,  the  pas- 
sage of  a  cannon-ball  would  produce  an  analogous  noise.  The  objec- 
tion is  not  well  founded,  for  the  cannon-ball  is  but  a  tortoise  in  compar- 
ison to  the  dart  of  the  lightning.  In  the  second  place,  the  sound  of 
thunder  may  be  due  to  the  fact  that  clouds  become  dilated  under  the 
influence  of  the  electric  tension  which  swells  them  in  a  certain  measure, 
lengthens  them,  and  stretches  them  with  so  much  force  at  certain  points 
that,  if  a  spark  causes  the  cloud  to  discharge,  the  outer  air,  being  no 
longer  retained  by  the  expansive  force  of  the  electric  fluid  in  equilibri- 
um with  it,  rushes  from  all  directions  toward  the  clouds.  To  this  may 
be  attributed  the  cause  of  thunder,  and  of  the  fall  of  rain  which  follows. 
The  electric  conditions  of  the  various  clouds  which  compose  a  storm 
being  dependent  the  one  upon  the  other,  the  discharge  of  one  must  lead 
to  that  of  several  others  more  or  less  distant.  In  the  one  case  as  in  the 
other,  the  sound  is  always  caused  by  the  expansion  of  the  air  at  the 
spot  where  the  more  or  less  partial  void  has  just  been  made,  as  hap- 
pens with  fire-arms,  the  bursting  of  a  bladder,  etc.  When  situated  at 
the  point  where  the  lightning  terminates — where  the  thunder-bolt  falls, 
according  to  the  vulgar  expression — this  noise  is  never  very  long,  and 
is  exactly  like  the  report  of  a  cannon,  a  fowling-piece,  or  a  pistol,  ac- 
cording to  the  intensity.  But  one  of  the  special  characteristics  of  thun- 
der consists  in  the  rolling,  as  its  name  imitates  it  in  every  language— 
thunder,  tonnerre,  tonitruum,  bronte,  donner. 

It  is  frequently  asked  to  what  this  rolling,  often  very  prolonged,  can 
be  due.  There  are  several  causes  for  it.  The  first  is  due  to  the  length 
of  the  flash,  and  to  the  difference  in  the  speed  of  sound  and  of  light. 
Let  us  imagine,  for  instance,  a  horizontal  flash  35,000  feet  long  and 

28 


434:  THE  ATMOSPHERE. 

3000  feet  high.  An  observer,  placed  beneath  one  extremity  of  the 
flash,  will  see  this  flash  in  its  full  length  for  an  instant ;  the  sound  will 
be  formed  at  the  same  moment  along  the  whole  line  of  the  flash ;  but 
the  sound-waves  will  reach  his  ears  at  different  times.  That  which 
starts  from  the  nearest  point  will  arrive  in  three  seconds,  as  sound  trav- 
els at  the  rate  of  about  1100  feet  per  second.  That  which  was  formed 
at  the  same  instant  at  a  point  6000  feet  distant  takes  twice  the  time 
to  arrive.  That  which  proceeds  from  a  point  at  13,200  feet  will  take 
twelve  seconds.  The  sound  formed  at  a  distance  of  35,000  feet  would 
take  thirty-three  seconds  to  travel ;  thus  the  rolling  will  continue  half 
a  minute,  gradually  becoming  fainter,  until  it  dies  away  altogether. 

If,  as  most  frequently  happens,  the  observer  is  not  situated  exactly  at 
one  of  the  extremities  of  the  flash,  but  at  a  certain  point  in  its  line  of 
passage,  he  hears  first  the  report,  which  gradually  grows  louder,  and 
then  diminishes.  In  this  case,  the  sound  starting  from  a  point  situated 
over  his  head,  and  at  a  height  of  1000  yards,  reaches  him  in  three  sec- 
onds; but  the  sounds  formed  on  either  side  at  equal  distances  arrive 
at  the  same  time  during  several  seconds,  and  sound  ceases  in  less  than 
thirty-two  seconds. 

To  this  cause  of  the  prolonged  rolling  must  be  "added  the  numerous 
discharges  which  often  take  place  very  rapidly  among  thunder-clouds 
— the  zigzags  and  ramifications  of  the  lightning,  caused  by  the  hygro- 
metrical  diversity  of  the  various  strata  of  air — the  echoes  repeated  by 
mountains,  the  soil,  waters,  and  the  clouds  themselves,  to  which  must 
further  be  added  the  interference  produced  by  the  encounter  of  differ- 
ent systems  of  sound-waves. 

The  duration  of  the  rolling  of  thunder  varies  very  much,  as  every 
one  may  have  remarked.  The  greatest  length  recorded  for  a  single 
flash  is  forty-five  seconds,  by  De  I'lsle^  at  Paris,  on  June  17,  1712. 
Upon  the  same  day  he  remarked  another,  which  lasted  forty-one  sec- 
onds; and  on  July  8,  in  the  same  year,  one  of  thirty-nine  seconds.  The 
intervals,  included  between  the  commencement  of  the  thunder  and  the 
different  phases  of  intensity  in  its  rolling,  were  as  follows  upon  this 
last  occasion  (July  8) : 

At  0  seconds,  flash  ; 

At  11  seconds,  slight  thunder ; 

At  12  seconds,  it  bursts ; 

At  32  seconds,  the  explosions  cease ; 

At  50  seconds,  the  sound  dies  gently  away. 


LIGHTNING  AND  THUNDER.  435 

The  intensity  of  thunder  varies  to  an  enormous  extent.  In  certain 
cases  it  has  been  compared  to  the  report  of  a  hundred  pieces  of  artillery 
discharged  at  the  same  time.  In  other  instances  the  report  is  no  louder 
than  that  of  a  pistol,  followed  by  a  rolling  sound  more  or  less  dull.  At 
times  the  explosions  remind  one  of  the  tearing  of  a  piece  of  silk,  at  oth- 
ers of  the  noise  made  by  a  cart  loaded  with  bars  of  iron  sent  loose 
down  a  steep  paved  street. 

The  longest  interval  ever  remarked  between  the  flash  and  the  report 
was  seventy-two  seconds.  This  was  at  Paris,  and  the  same  interval 
was  also  noticed  to  elapse  by  the  astronomer  De  1'Isle  on  April  30, 
1712.  In  these  two  cases  the  cloud  must  have  been  six  leagues  off. 
Next  to  these  exceptional  cases,  the  longest  interval  was  forty-nine 
seconds,  which  represents  ten  miles'  distance.  Direct  researches  have 
shown  that  a  storm  is  never  heard  at  a  greater  distance  than  thirteen 
miles,  rarely  at  more  than  seven  to  ten ;  the  flashes  are  visible,  but  the 
sound  does  not  travel  so  far.  The  fact  is  the  more  curious  as  cannon 
are  heard  at  a  much  greater  distance,  as  much  as  twenty-five  miles ;  and 
when  very  large,  they  may  be  heard  at  double  that  distance. 

Continued  cannonading,  as  during  a  siege  or  a  pitched  battle,  has 
been  heard  at  a  distance  of  thirty  leagues.  During  the  winter  of  1870, 
the  Krnpp  guns,  exhibited  in  Paris  in  1867,  were  heard  at  Dieppe,  a 
distance  of  eighty-four  miles,  during  the  bombardment  of  Paris.  The 
cannonade  of  March  30,  1814,  was  heard  at  Casson,  a  village  between 
Lisieux  and  Caen,  at  a  distance  of  forty-four  leagues  from  Paris.  Ara- 
go  relates  that  the  firing  at  Waterloo  was  audible  at  Creil,  120  miles 
distant.  Thus  the  thunder  manufactured  by  man  reaches  much  farther 
than  the  thunder  produced  by  nature.  If  thunder  is  not  audible  at 
more  than  six  leagues,  it  follows  that,  if  thunder  is  heard  with  the  sky 
clear,  the  report  must  be  produced  by  clouds  below  the  visible  horizon, 
as  we  can  not  see  beyond  six  leagues.  A  person  of  five  feet  five  inch- 
es in  height  is  able  to  see,  when  the  horizon  is  clear,  an  object  placed 
upon  the°ground  at  a  distance  of  13,000  feet.  If  the  object  in  question 
is  eighty  feet  high  in  the  air,  it  may  be  seen  at  five  and  a  half  leagues. 
If  it  is  1600  feet  high,  as  in  the  case  of  an  isolated  mountain,  it  will  be 
visible  at  a  distance  of  fifty  miles.  If  the  object  be  3300  feet  high,  as 
cumulus  clouds  are,  as  a  rule,  in  our  climates,  it  can  be  seen  at  a  dis 
tance  of  seventy  miles. 

For  a  thunder-clap  which  takes  place  when  the  sky  is  clear,  to 
produced  by  a  cloud,  we  must  consequently  suppose  the  cloud  to  be 


436  THE  ATMOSPHERE. 

less  than  100  feet  above  the  ground — a  state  of  things  never  witnessed. 
Thus  electricity  may  be  emitted  from  certain  regions  of  the  air,  from 
invisible  clouds,  and  may  produce  flashes  and  thunder-claps  during  fine 
clear  weather.  Observation  has  proved  this  to  be  a  fact,  but  one  of 
very  rare  occurrence. 

To  these  statements  bearing  upon  the  general  action  of  thun.der  and 
lightning,  I  may  add  that,  notwithstanding  the  extreme  rapidity  of  the 
flash,  it  has  been  found  possible  to  measure  its  duration,  which  does  not 
exceed  T^^nr  °f  a  second.  To  effect  this,  a  round  piece  of  card-board, 
divided  from  the  centre  to  the  circumference  into  black  and  white  sec- 
tions, is  made  use  of.  This  circle  is  made  to  turn  like  a  wheel,  with  a 
speed  equal  to  that  of  the  wind.  It  is  well  known  that  luminous  im- 
pressions remain  for  one-tenth  of  a  second  upon  the  retina.  Thus,  if  a 
hot  coal  is  turned  round,  and  if  the  revolution  is  made  in  one-tenth  of  a 
second,  and  as  each  successive  position  of  the  coal  remains  impressed 
for  the  same  length  of  time  upon  the  retina,  a  continuous  circle  becomes 
visible.  If  the  circular  piece  of  card-board,  with  its  black  and  white 
stripes,  is  made  to  revolve,  the  sectors  cease  to  be  visible,  and  we  can 
only  see  a  grayish  circle,  if  each  stripe  passes  before  our  eyes  in  less 
than  the  tenth  of  a  second.  But  it  is  possible  to  make  the  card-board 
revolve  more  than  a  hundred  times  in  a  second.  This  being  the  case, 
if  the  card-board  circle  is  exposed  to  a  continuous  light,  we  shall  be  un- 
able to  distinguish  the  lines,  inasmuch  as  they  come  before  our  eye 
much  more  rapidly  than  the  impression  which  they  produce  remains. 
But  if  the  circle  is  made  to  revolve  in  a  dark  place,  and  an  instantane- 
ous flash  of  light  suddenly  falls  upon  it,  and  as  suddenly  disappears,  the 
impression  produced  upon  our  eye  by  each  of  the  sectors  will  last  less 
than  one-tenth  of  a  second,  it  will  be  almost  instantaneous,  and  the  circle 
will  seem  to  us  to  be  motionless.  By  giving  this  apparatus  a  fixed  rate  of 
rotation,  it  has  been  ascertained  that  a  flash  lasts  but  Iojo0  of  a  second. 

Light,  traveling  a  distance  of  185,000  miles  in  a  second,  takes  but  an 
instant,  too  short  to  be  reckoned,  to  come  from  the  spot,  never  more 
than  a  few  miles  off,  at  which  the  flashes  are  produced.  Thus  we  see 
the  flash  at  the  very  moment  at  which  it  occurs.  But  sound  travels,  as 
we  have  seen  above,  less  rapidly — at  the  rate  of  1100  feet  a  second.  It 
follows  that  the  thunder-clap,  which  takes  place  at  the  same  time  as  the 
flash,  will  only  be  audible  to  us  ten  seconds  afterward  if  we  are  11,000 
feet  away  from  the  storm  ;  and  anyone  can,  therefore,  calculate  how  far 
off  the  storm  is  by  the  interval  between  the  flash  and  the  thunder. 


Fig.  Sl.-Harvesters  killed  by  lightning. 


LIGHTNING  AND  THUNDER.  439 

T  second  interval  corresponds  to 559  f^ 


2,200 

3        '  "  "  3,300 


1,100 


4,400  " 
6,500  " 
6,600  " 
7,700  " 


9,900 
11,000 
12,100 
13,200 


There  are  about  twelve  beatings  of  the  pulse  to  a  league.  When 
the  flash  extends  over  a  length  of  several  miles,  the  spot  struck  by  the 
thunder  may  be  very  distant,  although  the  report  is  heard  immediately 
after  the  flash,  because  it  is  the  sound  which  starts  from  the  nearest  ex- 
tremity of  the  flash  which  is  heard  first.  For  instance,  in  a  storm  on 
the  27th  of  June,  1866,  M.  Him  remarked  that  the  report  followed  im- 
mediately upon  the  flash,  although  this  same  flash  had  struck  down 
two  persons  beneath  a  tree  three  miles  distant. 

Many  are  the  marvelous  freaks  and  jests  played  by  electricity,  some- 
times ending  in  tragedy.  Among  the  most  remarkable  is  that  of  strik- 
ing a  person  dead,  and  leaving  him  in  the  exact  position  occupied  at 
the  moment  the  shock  was  given,  just  as  if  he  were  still  alive,  and  yet 
so  thoroughly  consumed  as  to  be  nothing  but  a  mass  of  cinders.  Thus 
we  are  told  that  at  Vic-sur-Aisne,  France,  in  1838,  three  soldiers 
sought  refuge  from  a  violent  thunder-storm  under  a  linden -tree. 
Some  peasants,  seeing  them  stand  motionless  long  after  the  storm  had 
passed,  and  receiving  no  response  to  a  pleasant  salutation,  touched 
them  on  the  shoulder.  The  bodies  instantly  crumbled  to  fine  ashes! 
Yet  the  moment  before  there  was  no  evidence  that  the  lightning  had 
touched  them.  Their  clothing  was  not  torn,  and  their  faces  wore  a 
natural  appearance.  The  following  remarkable  circumstance  was  wit- 
nessed by  Pastor  Butler :  On  the  27th  of  July,  1691,  ten  harvesters 
took  refuge  under  a  hedge  on  the  approach  of  a  thunder-storm.  The 
lightning  struck  and  killed  four  of  them,  who  remained  as  if  suddenly 
petrified.  One  of  them  was  just  putting  a  bit  of  tobacco  in  his  mouth, 
another  was  fondling  a  little  dog  on  his  knee  with  one  hand  and  feed- 
ing him  with  the  other.  M.  Cardan  relates  that  eight  harvesters,  tak- 


440 


THE  ATMOSPHERE. 


ing  their  noonday  repast  under  a  maple-tree  during  a  thunder-storm, 
were  killed  by  one  stroke  of  lightning.  When  approached  by  their 
companions,  after  the  storm  had  cleared  away,  they  seemed  to  be  still 
at  their  repast.  One  was  raising  a  glass  to  drink,  another  was  in  the 
act  of  taking  a  bit  of  bread,  a  third  was  reaching  out  his  hand  to  ? 
plate.  There  they  sat  as  if  petrified,  in  the  exact  position  in  which 
death  surprised  them. 


Fig.  82.— Curious  freak  of  lightning. 

On  the  10th  of,  September,  1845,  during  a  violent  thunder-storm,  a 
house  in  the  village  of  Salagnac,  France,  was  struck  by  lightning.  A 
large  ball  of  fire  descended  the  chimney,  and  rolled  across  the  floor  of  a 
room  in  which  sat  a  child  and  three  women.  No  one  was  hurt.  It 
then  rolled  out  through  the  centre  of  the  kitchen,  passing  close  to  the 
feet  of  a  young  peasant,  and  disappeared  through  a  crevice  in  the  wall. 
Its  erratic  course  ended  in  the  pig-sty,  the  harmless  occupant  of  which 
it  despitefully  slew,  without  setting  on  fire  the  straw  on  which  the  crea- 
ture lay. 


THE  SAINT  ELMO  FIRES  AND  THE  J AC K-W- LANTERNS. 


CHAPTER  III. 

THE  SAINT  ELMO  FIRES  AND  THE  JACK-O'-LANTERNS. 

THE  Saint  Elmo  fires  are  a  slow  manifestation  of  electricity,  a  quiet 
and  steady  outflow  (like  that  of  the  hydrogen  in  a  gas-burner),  which 
radiates  gently  over  the  topmost  points  of  lightning  conductors,  of 
buildings  and  vessels,  during  thunder  weather,  when  the  terrestrial  elec- 
tric tension  is  strongly  attracted  by  that  of  the  clouds. 

The  Saint  Elmo  fires  are  generally  seen  as  a  light  resting  on  the 
masts  of  ships.  The  following  are  some  of  the  most  recent  observa- 
tions made  : 

On  December  23,  1869,  in  latitude  46°  53'  north,  and  longitude  9° 
55'  west,  the  barometer  reading  29'61  inches,  thermometer  49°  1',  the 
log  of  the  packet  Imperatrice- Eugenie  records  the  occurrence  of  very 
violent  squalls.  Sharp  and  numerous  flashes  of  lightning  were  visible 
in  all  parts  of  the  horizon,  without  being  followed  by  a  single  clap  of 
thunder.  During  the  night  these  squalls  were  accompanied  by  heavy 
hailstorms,  and,  when  they  passed  over  the  vessel,  they  presented  the 
phenomenon  known  under  the  name  of  the  Saint  Elmo  fire. 

Luminous  tufts,  blue  in  color  and  about  a  foot  and  a  half  high,  ap- 
peared above  the  tips  of  the  conductors  upon  each  mast.  The  masts 
and  the  rigging  looked  phosphorescent,  and  the  tips  of  the  waves  also 
seemed  decked  with  tufts,  but  less  showy  than  those  that  appeared 
above  the  masts.  These  glimmerings  were  visible  whenever  the  squall 
reached  the  vessel.  Very  brilliant  when  the  wind  was  blowing  with  its 
full  force,  they  became  less  bright  as  it  fell,  and  disappeared  when  it 
dropped  altogether.  Only  those  parts  of  the  masts  and  the  rigging 
which  were  exposed  to  the  direct  action  of  the  squall  presented  this 
luminous  appearance.  They  looked  as  if  they  had  been  rubbed  with 
phosphorus.  The  phenomenon  did  not  take  place  upon  the  parts  which 
were  at  all  sheltered  from  the  wind,  nor  did  it  come  down  lower  than 
the  top-yards,  about  ninety  feet  above  the  level  of  the  sea.  The  phe- 
nomenon repeated  itself  several  times  during  the  night,  but  only  when 
the  squalls  were  accompanied  by  hail.  The  Saint  Elmo  fires  are  also 
seen  over  steeples.  The  following  is  one  of  the  most  recent  instances : 


442 


THE  ATMOSPHEBE. 


On  March  2, 1869,  these  flames  appeared  over  the  church  at  Sainte- 
Catherine-de-Fierbois,  in  the  canton  of  Sainte-Maure  and  the  arrondisse- 
ment  of  Chinon ;  no  thunder  was  audible  during  the  storm,  and  the 
steeple  disarmed  the  thunder-clouds.  A  correspondent  of  the  French 
Scientific  Association  wrote  as  follows :  "  Toward  the  end  of  the  tem- 
pest, when  the  wind  had  somewhat  abated  and  the  rain  was  not  so 
heavy,  several  persons  remarked  a  crown  of  fire  around  the  cross  that 
surmounted  the  steeple  of  the  church,  about  130  feet  high.  One  of  the 
eye-witnesses  saw  it  for  at  least  five  minutes  (he  did  not  perceive  it 

begin);  the  light  was  so  bright 
that  the  steeple  and  cross  were 
as  plain  to  the  eye  as  in  full 
daylight;  the  light  finally  died 
away  like  that  of  a  burned- 
out  candle,  without  the  least 
change  of  position." 

Luminous  tufts  of  electrici- 
ty have  often  been  seen  above 
the  spire  of  Notre-Dame  dur- 
ing certain  violent  thunder- 
storms of  a  summer  evening. 

The  Saint  Elmo  fires  are  oc- 
casionally seen  playing  over 
man  himself,  over  his  clothes, 
or  any  object  that  he  has  in 
his  hand. 

Julius  Csesar  relates  how  in 
the  month  of  February,  about 
the  second  watch  of  the  night, 
a  thick  cloud  suddenly  arose, 

Fig.  83.— Saint  Elmo  fire  over  the  spire  of  Notre-Dame,    followed  by  a  shower  of  Stones ; 

and  that  during  the  same  night 
the  pike-heads  of  the  fifth  legion  seemed  to  be  on  fire. 

According  to  Procopius,  a  similar  phenomenon  was  seen  over  the 
pikes  and  lances  of  Belisarius's  army  in  the  war  with  the  Yandals. 

Livy  states  that  the  pikes  of  some  soldiers  in  Sicily,  and  a  whip 
which  a  horseman  in  Sardinia  had  in  his  hand,  seemed  as  if  on  fire. 
Even  the  coats  of  mail  were  luminous  and  bright  with  numerous  flames 
of  fire. 


THE  SAINT  ELMO  FIRES  AND  THE  JACK-O'-LANTERNS.  443 

When  in  1769,  in  the  midst  of  a  violent  storm,  bright  tufts  ap- 
peared over  the  cross  upon  the  steeple  at  Hohen-Gebrachim,  two  per- 
sons, who  had  come  to  put  out  the  conflagration,  as  they  thought, 
were  at  once  surprised  and  terrified  to  see  their  heads  covered  with 
fire  and  light. 

On  May  8,  1831,  after  sunset,  the  whole  atmosphere  was  on  fire,  pre- 
saging a  violent  storm.  At  the  extremity  of  the  flag-staff  at  Algiers 
there  appeared  a  white  light  in  the  shape  of  a  brush  which  lasted  for 
half  an  hour.  Some  artillery  and  engineer  officers  were  walking  upon 
the  terrace  of  Fort  Bab-Azoun,  and  each  noticed,  to  his  surprise,  that 
the  heads  of  his  companions  were  tipped  by  small  luminous  tufts. 
When  they  raised  their  hands,  brushes  of  light  formed  at  the  tips  of 
their  fingers. 

In  some  instances  the  Saint  Elmo  fires  have  been  noticed  in  the  shape 
of  flames ;  at  other  times  a  man's  whole  body  has  been  seen  radiant 
with  light.  Peytier  and  Hossard  were  frequently  enveloped,  in  the 
Pyrenees,  in  centres  of  storms,  which  seemed  so  formidable  as  seen  from 
the  plains  below,  that  the  spectators  believed  they  must  have  perished 
in  them.  On  several  occasions  their  hair  and  the  tassels  of  their  caps 
stood  upright  and  emitted  a  bright  light,  accompanied  by  a  loud  hiss- 
ing noise. 

Letestu,  in  1786,  remained  for  three  hours  of  the  night  in  his  balloon 
during  a  storm;  he  heard  a  deafening  noise;  the  car  was  filled  with 
snow  and  hail,  and  the  gilding  upon  his  flag  emitted  scintillations. 

The  discharge  of  electricity  from  the  soil  into  the  atmosphere  is  some- 
times accompanied  by  remarkable  phenomena — by  a  kind  of  electric 
hum  upon  the  summits  of  mountains. 

These  various  phenomena  are  due  solely  to  disengagements  of  elec- 
tricity. We  must  not  confound  with  the  Saint  Elmo  fires  gleams  of 
light  which  resemble  them  very  much,  viz.,  the  ignes-fatui.  These  lat- 
ter are  not  caused  by  electricity. 

The  ignes-fatui,  or  will-o'-the-wisp,  is  a  wandering  and  shadowy  fire, 
produced  by  the  emanations  of  phosphureted  hydrogen  gas,  which  rises 
out  of  places  where  vegetable  and  animal  substances  are  in«  process  of 
decomposition,  such  as  cemeteries,  manure-heaps,  or  marshes,  and  which 
become  spontaneously  inflamed  when  combined  with  the  oxygen  of 
the  air. 

These  vacillating  lights  have  appealed  to  the  superstitious  feelings  of  , 
the  people.     The  frightened  imagination  has  often  looked  upon  them  as 


444  THE  ATMOSPHERE. 

wandering  spirits,  and  they  have  often  terrified  those  who  have  seen 
them  gliding  between  the  graves  of  a  church-yard  during  the  silence 
of  night. 

They  are  sometimes  emitted  suddenly  when  an  old  burying-vault  is 
opened ;  and  as  in  former  days  lighted  lamps  were  placed  in  the  graves, 
the  credulous  believed  they  were  inextinguishable. 


AURORA  BOREA.LES. 


CHAPTER  IV. 

AURORA    BOREALES. 

WE  now  come  to  the  most  curious  and  the  grandest  of  the  various 
manifestations  of  electricity  in  the  atmosphere.  As  we  have  seen,  the 
globe  is  one  vast  reservoir  for  this  subtle  fluid,  which  exists  in  all  the 
worlds  appertaining  to  our  system,  and  of  which  the  radiating  focus  is 
in  the  sun  itself.  Like  attraction,  light,  and  heat,  electricity  is  a  general 
power  in  nature.  Its  palpitations  sustain  the  life  of  the  universe,  and 
even  upon  our  planet  currents  of  it  are  in  constant  circulation  from  the 
equator  to  the  poles,  and  from  the  poles  to  the  equator.  The  delicate 
magnetic  needle  and  the  sea-compass  indicate  this  perpetual  circulation 
as  moving  northward.  The  magnetic  needle  oscillates  and  becomes 
agitated  when  these  disturbances  become  violent  and  there  are  great 
changes  in  its  position.  The  lightning  which  falls  upon  a  ship  often 
exercises  an  ineffaceable  influence  upon  the  compass;  and  while  the 
pilot  assumes  that  the  needle  is  still  pointing  north,  he  runs  the  risk  of 
being  driven  on  to  the  rocks  of  some  unknown  shore.  If  a  bright  au- 
rora borealis  is  shining  over  Stockholm  or  Reikjavik,  the  compass  in  the 
Paris  Observatory,  hundreds  of  leagues  off,  is  affected  by  it,  and  seems 
as  if  it  were  asking  the  editor  of  the  Bulletin  International  to  see  what 
was  the  matter. 

The  aurora  borealis  is  one  of  the  grand  results  of  atmospheric  elec- 
tricity. Instead  of  a  furious  and  violent  storm  limited  to  a  few  leagues, 
it  is  a  gentle  and  gradual  recomposition  of  the  negative  fluid  of  the 
earth  with  the  positive  fluid  of  the  atmosphere,  taking  place  in  the 
aerial  heights,  in  the  upper  hydrogenous  atmosphere.  This  disengage- 
ment of  electricity  in  a  vast  sheet  is  only  visible  at  night,  and  assumes 
every  imaginable  kind  of  shape,  according  to  the  way  in  which  it  takes 
place,  and  to  the  perspective  caused  by  the  distance  of  the  observer.  At 
one  time  the  eye  may  scarcely  have  time  to  catch  its  rapid  undulations, 
alternately  rose-colored  and  white  in  hue,  as  they  dart  across  the  sky. 
Now  it  takes  the  shape  of  a  cloth  of  gold  and  purple,  which  seems  to 
fall  from  the  celestial  heights ;  now  it  is  a  fiery  dew,  accompanied  by  a 
strange,  rustling  sound,  or  it  may  appear  in  the  form  of  sheaves  of 


446  THE  ATMOSPHERE. 

flame,  darting  from  the  north  to  the  various  points  of  the  compass.  It 
is  principally  in  the  neighborhood  of  the  polar  circles,  where  thunder- 
storms are  rare,  that  these  manifestations  of  terrestrial  electricity  are  seen 
to  the  fullest  advantage.  Michelet,  who  describes  so  graphically  the 
great  phenomena  of  nature,  speaks  of  the  aurora  borealis  in  this  way : 

"  The  pole  seems  a  kingdom  of  death.  But,  in  reality,  general  life  is 
triumphant  there.  The  two  spirits  of  the  globe  (magnetic  and  electric) 
make  their  nightly  rejoicing  in  this  desert." 

"The  aerial  currents,  and  the  currents  of  the  sea,  are  their  vehicles. 
The  two  torrents  of  heated  waters  which,  from  Java  and  Cuba,  travel 
northward,  where  they  cool  and  freeze,  and  then  return  refreshed  to  the 
centre  whence  they  started,  both  assist  in  keeping  up  the  magnetic  and 
electric  correspondence  between  the  equator  and  the  pole.  Their  storms 
are  dependent  upon  each  other.  In  summer,  when  the  melted  ice  from 
the  poles  and  the  northern  currents  make  their  cooling  influence  felt. 
the  magnetic  element  seems  to  extend  in  the  direction  of  the  central 
electricity;  hence  the  violent  storms,  especially  those  near  to  this 
centre." 

Spitzbergen  is  a  very  favorable  region  for  witnessing  an  aurora  bo- 
realis. In  a  voyage  undertaken  in  1839,  M.  Ch.  Martins  observed  and 
analyzed  a  large  number,  which  he  describes  thus  (see  "Le  Tour  du 
Monde,"  1865,  vol.  ii.,  p.  10): 

"At  times  they  are  simple  diffused  gleams  or  luminous  patches;  at 
others  quivering  rays  of  pure  white  which  run  across  the  sky,  starting 
from  the  horizon  as  if  an  invisible  pencil  were  being  drawn  over  the 
celestial  vault;  at  times  it  stops  in  its  course,  the  incomplete  rays  do 
not  reach  the  zenith,  but  the  aurora  continues  at  some  other  point;  a 
bouquet  of  rays  darts  forth,  spreads  out  into  a  fan,  then  becomes  pale 
and  dies  out.  At  other  times  long  golden  draperies  float  above  the 
head  of  the  spectator,  and  take  a  thousand  folds  and  undulations,  as  if 
agitated  by  the  wind.  They  appear  to  be  but  at  a  slight  elevation  in 
the  atmosphere,  and  it  seems  strange  that  the  rustling  of  the  folds,  as 
they  double  back  on  to  each  other,  is  not  audible.  Generally  a  lumi- 
nous bow  is  seen  in  the  north  ;  a  black  segment  separates  them  from 
the  horizon,  its  .dark  color  forming  a  contrast  with  the  pure  white  or 
bright  red  of  the  bow  which  darts  forth  the  rays,  extends,  becomes  di- 
vided, and  soon  presents  the  appearance  of  a  luminous  fan,  which  fills 
the  northern  sky,  mounts  nearly  to  the  zenith,  where  the  rays,  uniting, 
form  a  crown,  which,  in  its  turn,  darts  forth  luminous  jets  in  all  direc- 


AURORA  BORE  ALES. 


447 


tions.  The  sky  then  looks  like  a  cupola  of  fire:  the  blue,  the  green, 
the  yellow,  the  red,  and  the  white  vibrate  in  the  palpitating  rays  of 
the  aurora.  But  this  brilliant  spectacle  lasts  only  a  few  minutes;  the 
crown  first  ceases  to  emit  luminous  jets,  and  then  gradually  dies  out; 
a  diffuse  light  fills  the  sky ;  here  and  there  a  few  luminous  patches,  re- 
sembling light  clouds,  open  and  close  with  an  incredible  rapidity,  like 
a  heart  that  is  beating  fast.  They  soon  get  pale  in  their  turn ;  every 


Fig.  84. — An  aurora  borealis  over  the  Polar  Sea. 


thing  fades  away  and  becomes  confused ;  the  aurora  seems  to  be  in  its 
death-throes;  the  stars,  which  its  light  had  obscured,  shine  with  a  re- 
newed brightness;  and  the  long  polar  night,  sombre  and  profound, 
again  assumes  its  sway  over  the  icy  solitudes  of  earth  and  ocean."  In 
presence  of  such  phenomena,  the  poet  and  the  artist  are  compelled  to 
confess  their  littleness — the  savant  alone  does  not  despair.  After  hav- 
ing admired  the  spectacle,  he  studies,  analyzes,  compares,  and  discusses 


448  THE  ATMOSPHERE. 

it ;  he  succeeds  in  proving  that  these  aurorae  are  due  to  electric  radia- 
tions from  the  poles  of  the  earth,  which  is  a  colossal  magnet,  the  north- 
ern pole  of  which  is  situated  to  the  north  of  North  America,  not  far 
from  the  pole  of  our  hemisphere,  while  its  southern  pole  is  in  the  sea  to 
the  south  of  Australia,  near  Victoria." 

A  few  instances  will  suffice  to  prove  the  electro-magnetic  nature  of 
the  aurora  borealis.  At  Spitzbergen.  a  magnetic  needle  suspended 
horizontally  by  an  untwisted  piece  of  silk-thread  is  turned  toward  the 
west.  As  soon  as  the  aurora  begins,  the  person  observing  this  needle 
remarks  that,  instead  of  being  sensibly  motionless,  it  is  agitated,  pass- 
ing to  and  fro  from  right  to  left,  and  from  left  to  right  In  proportion 
as  the  aurora  becomes  more  brilliant,  the  agitation  of  the  needle  in- 
creases, and  the  observer  is  able  to  judge  of  the  intensity  of  the  aurora 
by  the  motions  of  the  needle  without  leaving  his  study.  Lastly,  when 
the  corona  is  formed  in  the  sky,  its  centre  will  be  found  exactly  in  the 
direction  to  which  a  magnetic  needle,  hanging  freely,  points.  The 
aurorae  boreales  are  therefore  intimately  connected  with  the  magnetic 
phenomena  of  the  terrestrial  globe. 

What  a  strange  world  is  that  of  the  poles!  Nearly  every  night 
there  is  a  more  or  less  brilliant  display  of  these  auroral  lights;  from 
the  middle  of  January,  there  is  an  hour's  twilight  at  noon ;  the  aurora, 
announcing  the  return  of  the  sun,  becomes  grander  as  it  mounts  toward 
the  zenith.  Lastly,  on  the  16th  of  February,  a  segment  of  the  solar 
disk,  resembling  a  luminous  point,  shines  brightly  for  an  instant,  and 
as  rapidly  disappears;  but  every  day  at  noon  the  segment  increases, 
until  the  whole  orb  rises  above  the  sea :  it  is  the  end  of  the  long  win- 
ter night;  after  that,  day  and  night  follow  each  other  for  sixty-five 
days,  until  the  21st  of  April,  when  begins  day-time,  lasting  four  months, 
during  which  period  the  sun  revolves  above  the  horizon,  gradually  be- 
coming lower,  and  finally  disappearing. 

In  North  America,  to  the  east  of  Beh ring's  Straits,  there  is  a  large 
tract  of  territory  little  known  to  Frenchmen — Alaska — which  is  trav- 
ersed by  the  Arctic  Circle.  It  formed  part  of  Eussian  America  a  few 
years  ago,  and  was  45,000  square  leagues  in  extent.  It  was  purchased 
by  the  United  States  in  October,  1867.  In  a  curious  account  of  a  voy- 
age which  Frederick  Whymper  made  there,  in  1865  (see  "Le  Tour  du 
Monde,"  1869,  vol.  ii.,  p.  247),  there  is  recorded  the  observation  of  that 
very  rare  phenomenon,  viz.,  an  aurora  borealis  in  the  shape  of  a  ribbon, 
extending  in  undulating  folds  in  the  heights  of  the  air. 


BORE  ALES. 


Fig.  85.— Aurora  borealis  observed  at  Bossekop  (Spitzbergen)  January  6, 1839. 

To  use  the  traveler's  own  words,  "  It  was  on  the  27th  of  December, 
as  we  were  about  to  retire  for  the  night,  that  we  were  informed  that 
an  aurora  borealis  was  visible  in  the  west  We  at  once  climbed  the 
roof  of  the  highest  building  in  the  fort,  in  order  to  contemplate  this 
splendid  phenomenon.  It  was  not  in  the  form  of  an  arch,  as  often  is 
the  case,  but  the  light  was  serpentine-shaped  and  undulating,  the  form 
and  the  color  varying  every  instant,  being  at  one  moment  of  a  pale  and 
soft  tint  like  moonbeams,  while  at  the  next  long  bands  of  blue,  rose, 
and  violet  stood  out  upon  the  silvery  background.  The  scintillations 
extended  from  the  lower  extremity  upward,  and  their  brightness  be- 
came fused  with  that  of  the  stars,  the  brilliancy  of  which  was  visible 
through  the  spiral  vapor." 

29 


450  THE  ATMOSPHERE. 

Auroras  boreales  are  rather  rare  in  France,  and  a  whole  lifetime  may 
pass  without  an  opportunity  being  afforded  of  witnessing  one  which  is 
in  the  least  degree  approaching  to  perfection.  Three  of  these  phenom- 
ena, however,  and  very  beautiful  of  their  kind,  were  seen,  viz.,  on  the 
15th  of  April  and  the  13th  of  May,  1869 ;  and  on  the  24th  of  October, 
1870. 

The  first  was  witnessed  by  M.  Silbermann  at  the  College  de  France, 
by  M.  Chapelas-Coulvier-Gravier  at  the  Luxembourg,  and  by  M.  Treme- 
schini  at  Belleville.  It  was,  to  a  certain  extent,  double.  The  first  part 
made  its  appearance  at  ten  minutes  after  eight  in  the  form  of  a  large 
bundle  of  luminous  columns,  reddish  in  hue,  spreading  from  the  point- 
ers of  the  Great  Bear  toward  the  east,  like  a  fan.  The  background  of 
the  sky  was  at  this  point  also  tinted  with  a  reddish  light.  The  second 
part  came  at  half-past  ten.  Eays  were  emitted  from  a  small  luminous 
bow  that  appeared  in  the  north.  These  rays,  of  a  very  decided  green- 
ish hue  at  the  lower  base,  were,  on  the  contrary,  at  their  upper  extremi- 
ty of  a  splendid  purple.  At  certain  moments  the  aspect  of  the  phe- 
nomenon changed,  the  light  became  agglomerated  at  certain  points, 
forming  dense  masses  or  patches,  very  brilliant,  white  in  the  centre  of 
the  aurora,  and  blood-red  at  the  circumference.  An  immense  number 
of  luminous  streaks,  parallel  with  each  other,  traversed  the  band  in  the 
direction  of  the  magnetic  meridian.  The  phenomenon  lasted  half  an 
hour,  with  several  variations  of  intensity. 

Aurorse  boreales  occur  at  very  different  elevations.  According  to 
Bravais's  measurements,  the  ordinary  height  is  between  60  and  120 
miles.  Loomis  avers  that  the  extreme  point  from  whence  the  flashes 
are  darted  is  as  much  as  400  or  even  500  miles !  If  this  be  so,  they 
must  occur  in  the  upper  atmosphere,  of  which  I  have  treated  in  the 
early  part  of  the  present  work.  At  the  same  time  some  have  been 
seen  at  a  much  lower  elevation,  as  low  down  as  the  clouds.  Their  ex- 
tent also  varies  very  much.  Thus,  by  a  letter  from  Ireland,  I  gather 
that  a  very  brilliant  aurora  was  seen  at  Cork  on  September  11,  1871, 
at  10  P.M.  It  was  not  visible  in  Paris,  which  is  only  480  miles  distant. 
An  aurora  seen  at  Cherbourg  on  the  19th  of  February,  1852,  was  not 
visible  at  Paris,  though  the  distance  is  only  180  miles.  E.  Liais  says 
that  it  could  not  be  at  a  height  of  more  than  23,000  feet.  Upon  the 
other  hand,  there  are  auroras  which  extend  over  an  immense  space. 
That  of  September  3, 1839,  was  seen  both  in  America  and  Europe,  as 
also  that  of  January  5,  1769.  That  of  September  2,  -1859,  was  visible 


••iififiw 

I 


AURORA  BOREALES.  453 

from  New  York  to  Siberia,  and  from  both  hemispheres,  at  the  Cape 
of  Good  Hope,  in  Australia,  Salvador,  Philadelphia,  and  Edinburgh ! 
This  was  the  first  time  that  the  eye  verified  what  theory  had  advanced, 
viz.,  that  aurorse  boreales  and  the  southern  aurora  occur  at  the  same 
time  in  the  two  hemispheres  under  the  influence  of  the  same  current. 
The  extremities  of  the  globe  are  brought  into  intimate  relation  with 
each  other  by  the  fluid  which  circulates  incessantly  in  the  air  and  upon 
the  soil.  At  certain  solemn  moments,  magnetism  augments  in  intensi- 
ty, and  seems  to  reanimate  the  life  of  our  planet 

The  production  of  aurorae  boreales  is,  in  Humboldt's  opinion,  one  of 
the  most  striking  proofs  of  the  faculty  which  our  planet  possesses  of 
emitting  light.  He  says,  "It  results  from  the  phenomenon  of  aurorae, 
that  the  earth  is  endowed  with  the  property  of  emitting  a  light  distinct 
from  that  of  the  sun.  The  intensity  of  this  light  is  rather  greater  than 
that  of  the  moon  in  its  first  quarter.  It  is  at  times  (January  7, 1831) 
strong  enough  to  admit  of  one's  reading  printed  characters  without 
difficulty.  This  light  of  the  earth,  the  emission  of  which  toward  the 
poles  is  almost  continuous,  reminds  us  of  the  light  of  Venus,  the  part  of 
which,  not  lighted  by  the  sun,  often  glimmers  with  a  dim  phosphores- 
cent light.  Other  planets  may  also  possess  a  light  evolved  out  of  their 
own  substance.  There  are  other  instances  in  our  atmosphere  of  this  pro- 
duction of  terrestrial  light,  such  as  the  celebrated  fogs  of  1783  and  1831, 
which  emitted  a  perceptible  light  during  the  night.  Such,  too,  are  those 
large  clouds  which  are  brilliant  with  a  steady  and  motionless  light;  and 
such,  too,  as  Arago  has  truly  remarked,  is  that  diffuse  light  which  guides 
our  steps  during  the  nights  of  spring  and  autumn,  when  the  clouds  in- 
tercept all  celestial  light,  and  snow  does  not  cover  the  ground." 

I  may  further  remark  that  aurora  boreales  are  more  or  less  period- 
ical. They  were  very  numerous  in  Belgium  and  Western  Europe  dur- 
ing the  last  half  of  the  eighteenth  century.  They  were  very  rare  in 
the  seventeenth,  and  very  frequent  in  the  sixteenth  century.  This  sec- 
ular periodicity  seems  to  be  of  about  a  century  and  a  half.  There  is 
a  monthly  variation,  more  accurately  ascertained.  They  are  most  fre- 
quent about  the  time  of  the  equinoxes,  and  seem  to  be  seven  times 
more  numerous  in  March  and  October  than  in  June. 

Such  are  the  last  and  the  grandest  of  the  phenomena  which  we  have 
to  contemplate  in  this  gallery  of  the  works  of  the  Atmosphere. 

THE    END. 


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ADDISON'S  COMPLETE  WORKS.  The  Works  of  Joseph  Addison,  embracing  the 
whole  of  the  "  Spectator."  Complete  in  3  vols.,  Svo,  Cloth,  $6  00. 

ALCOCK'S  JAPAN.  The  Capital  of  the  Tycoon:  a  Narrative  of  a  Three  Years- 
Residence  in  Japan.  By  Sir  RUTHERFORD  ALCOOK,  K.C.B.,  Her  Majesty's  Envoy 
Extraordinary  and  Minister  Plenipotentiary  in  Japan.  With  Maps  and  Engravings'. 
2  vols.,  12mo,  Cloth,  $3  50. 

ALISON'S  HISTORY  OF  EUROPE.  FIRST  SERIES  :  From  the  Commencement  of 
the  French  Revolution,  in  1789,  to  the  Restoration  of  the  Bourbon?,  in  1815.  [  lu 
addition  to  the  Notes  on  Chapter  LXXVI.,  which  correct  the  errors  of  the 
original  work  concerning  the  United  States,  a  copious  Analytical  Index  has  been 
appended  to  this  American  edition.]  SECOND  SERIES  :  From  the  Fall  of  Napoleon, 
in  1815,  to  the  Accession  of  Louis  Napoleon,  in  1852.  8  vols.,  Svo,  Cloth,  $16  00. 

BALDWIN'S  PRE-HISTORIC  NATIONS.  Pre-Historte  Nations :  or,  Inquiries  con- 
cerning some  of  the  Great  Peoples  and  Civilizations  of  Antiquity,  and  their 
Probable  Relation  to  a  still  Older  Civilization  of  the  Ethiopians  or  Cushites  of 
Arabia.  By  JOHN  D.  BALDWIN,  Member  of  the  American  Oriental  Society. 
12mo,  Cloth,  $1  75. 

BARTH'S  NORTH  AND  CENTRAL  AFRICA.  Travels  and  Discoveries  in  North 
and  Central  Africa :  being  a  Journal  of  an  Expedition  undertaken  under  the 
Auspices  of  H.  B.  M.'s  Government,  in  the  Years  1849-1855.  By  HENRY  BABTII, 
Ph.D.,  D.C.L.  Illustrated.  3  vols.,  Svo,  Cloth,  $12  00. 

HENRY  WARD  BEECHER'S  SERMONS.  Senrtons  by  HENRY  WARD  BEKCHKB, 
Plvmouth  Church,  Brooklyn.  Selected  from  Published  and  Unpublished  Dis- 
courses, and  Revised  by  their  Author.  With  Steel  Portrait.  Complete  In  2  vols., 
8ro,  Cloth,  $5  00. 

LYMAN  BEECHER'S  AUTOBIOGRAPHY,  &a  Autobiography,  Correspondence, 
Ac.,  of  Lyman  Beecher,  D.D.  Edited  by  his  Son,  CHARLES  BEKOHES.  With  Three 
Steel  Portraits,  and  Engravings  on  Wood.  In  2  vols.,  12mo,  Cloth,  $5  00. 

BOSWELL'S  JOHNSON.  The  Life  of  Samuel  Johnson.  LL.D.  Including  a  Journey 
to  the  Hebrides.  By  JAMES  BOSWELL,  Esq.  A  New  Edition,  with  mimeron; 
Additions  and  Notes.  By  JOHN  WILBON  CBOKKB,  LL.D.,  F.R.S.  Portrait  of 
Boswell.  2  vols.,  Svo.  Cloth,  $4  00. 


4      Harper  &*  Brothers'  Valu-able  and  Interesting  Works. 


DRAPER'S  CIVIL  WAR.  History  of  the  American  Civil  War.  By  JOHN  W.  DRA- 
PEK,  M.D.,  LL.D.,  Professor  of  Chemistry  and  Physiology  in  the  University  of 
New  York.  In  Three  Vols.  8vo,  Cloth,  $3  50  per  vol. 

DRAPER'S  INTELLECTUAL  DEVELOPMENT  OP  EUROPE.  A  History  of  the 
Intellectual  Development  of  Europe.  By  JOHN  W.  DEAPF.B,  M.D.,  LL.D.,  Profess- 
or of  Chemistry  and  Physiology  in  the  University  of  New  York.  8  vo,  Cloth,  $5  00 

DRAPER'S  AMERICAN  CIVIL  POLICY.  Thoughts  on  the  Future  Civil  Policy  of 
America.  By  JOHN  W.  DRAPER,  M.D.,  LL.D.,  Professor  of  Chemistry  and  Physiol- 
ogy in  the  University  of  New  York.  Crown  8vo,  Cloth,  $2  50. 

DU  CHAILLU'S  AFRICA.  Explorations  and  Adventures  in  Equatorial  Africa  with 
Accounts  of  the  Manners  and  Customs  of  the  People,  and  of  the  Chase  of  the  Go- 
rilla, the  Crocodile,  Leopard,  Elephant,  Hippopotamus,  and  other  Animals.  By 
PAUL  1$.  Du  CHAILLU.  Numerous  Illustrations.  8vo,  Cloth,  $5  00. 

BELLOWS'S  OLD  WORLD.  The  Old  World  in  its  New  Face:  Impressions  of  Eu- 
rope in  1867-1868.  By  HENRY  W.  BELLOWS.  2  vols.,  12mo,  Cloth,  $3  50. 

BROD  HEAD'S  HISTORY  OF  NEW  YORK.  History  of  the  State  of  New  York. 
By  JOHN  ROMEYN  BRODHEAD.  1609-1691.  2  vols.  8vo,  Cloth,  $3  00  per  vol. 

BROUGHAM'S  AUTOBIOGRAPHY.  Life  and  Times  of  HENRY,  LORD  BROUGHAM. 
Written  by  Himself.  In  Three  Volumes.  12mo,  Cloth,  $2  00  per  vol. 

BULWER'S  PROSE  WORKS.     Miscellaneous  Prose  Works  of  Edward  Bulwer. 

Lord  Lytton.    2  vols.,  12mo,  Cloth,  $3  50. 
BULWER'S  HORACE.    The  Odes  and  Epodes  of  Horace.    A  Metrical  Translation 

into  English.    With  Introduction  and  Commentaries.    By  LOEI>  LYTTON.    With 

Latin  Text  from  the  Editions  of  Orelli,  Macleane,  and  Yonge.    12mo,  Cloth,  $1  75. 
BULWER'S  KING  ARTHUR.    A  Poem.    By  EARL  LYTTON.    New  Edition.    12mo, 

Cloth,  $1  75. 

BURNS'S  LIFE  AND  WORKS.  The  Life  and  Works  of  Robert  Burns.  Edited 
by  ROBERT  CHAMBERS.  4  vols.,  12mo,  Cloth,  $6  00. 

REINDEER,  DOGS,  AND  SNOW-SHOES.  A  Journal  of  Siberian  Travel  and  Ex- 
plorations made  in  the  Years  1865-'67.  By  RICUAUI>  J.  BUSH,  late  of  the  Russo- 
American  Telegraph  Expedition.  Illustrated.  Crown  8vo,  Cloth,  $3  00. 

CARLYLE'S  FREDERICK  THE  GREAT.  History  of  Friedrich  II.,  called  Frederick 
the  Great.  By  TIIOMAS  CARLYLE.  Portraits,  Maps,  Plans.  &c.  6  vols.,  12mo, 
Cloth,  $12  00. 

OARLYLE'S  FRENCH  REVOLUTION.  History  of  the  French  Revolution.  Newly 
Revised  by  the  Author,  with  Index,  &c.  2  vols.,  12mo,  Cloth,  $3  50. 

CARLYLE'S  OLIVER  CROMWELL.  Letters  and  Speeches  of  Oliver  Cromwell. 
With  Elucidations  and  Connecting  Narrative.  2  vols.,  12mo,  Cloth,  $3  50. 

CHALMERS'S  POSTHUMOUS  WORKS.  The  Posthumous  Works  of  Dr.  Chalmers. 
Edited  by  his  Sou-in-Law,  Rev.  WILLIAM  HANNA,  LL.D.  Complete  in  9  vols., 
12mo,  Cloth,  $13  50. 

COLERIDGE'S  COMPLETE  WORKS.  The  Complete  Works  of  Samuel  Taylor 
Coleridge.  With  an  Introductory  Essay  upon  his  Philosophical  and  Theological 
Opinions.  Edited  by  Professor  SHEDD.  Complete  in  Seven  Vols.  With  a  fine 
Portrait.  Small  8vo,  Cloth,  $10  50. 

DOOLITTLE'S  CHINA.  Social  Life  of  the  Chinese :  with  some  Account  of  their  Re  - 
ligious,  Governmental,  Educational,  and  Business  Customs  and  Opinions.  With 
special  but  not  exclusive  Reference  to  Fuhchau.  By  Rev.  JUSTUS  DOOLITTLE, 
Fourteen  Years  Member  of  the  Fuhchau  Mission  of  the  American  Board.  Illus- 
trated with  more  than  150  characteristic  Engravings  on  Wood.  2  vols..  12ino 
Cloth,  $5  00. 

GIBBON'S  ROME.  History  of  the  Decline  and  Fall  of  the  Roman  Empire.  By  EL- 
WARD  GIBBON.  With  Notes  by  Rev.  H.  H.  MILMAN  and  M.  GUIZOT.  A  new  cheap 
Edition.  To  which  is  added  a  complete  Index  of  the  whole  Work,  and  a  Portrait 
of  the  Author.  6  vols.,  12mo,  Cloth,  $9  00. 

HAZEN'S  SCHOOL  AND  ARMY  IN  GERMANY  AND  FRANCE.  The  School 
and  the  Army  in  Germany  and  France,  with  a  Diary  of  Siege  Life  at  Versailles. 
By  Brevet  Mnjor-General  W.  B.  HAZEN,  U.S.A.,  Colonel  Sixth  Infantry.  Crown 
Svo,  Cloth,  $•;  so. 


Harper  6-  Brothers'  Valuable  and  Interesting  Works.      5 

HARPER'S'  NEW  CLASSICAL  LIBRARY.    Literal  Translations. 

The  following  Volumes  are  now  ready.  Portraits.  12mo,  Cloth,  $1  50  each. 
CAESAR.— VIRGIL.  — SALLUST.—  HORACE.- CICERO'S  ORATIONS.— CICERO'S  Orviocsi 
&c.— CIOERO  ON  ORATORY  ANI.  ORATORS. -TACITUS  (2  vols.).  _  TERENCE  — 
SOPUOOI.ES.— JUVENAL.— XENOPHON.—  HOMER'S  ILIAH — HOMKB'B  OHYSSKY.— 
HKRODOTUS.— DEMOSTHENES.— Tuucvuir>Ea.—^EsouYLU8.— EURIPIDES  (2  vols.) 
— LIVY  (2  vols.). 

DA  VIS'S  CARTHAGE.  Carthage  and  her  Remains :  being  an  Account  of  the  Exca 
vations  and  Researches  on  the  Site  of  the  Phoenician  Metropolis  in  Africa  and  other 
adjacent  Places.  Conducted  under  the  Auspices  of  Her  Majesty's  Government. 
Bv  Dr.  DAVIS,  F.R.G.S.  Profusely  Illustrated  with  Maps,  Woodcuts,  Chromo- 
Lithographs,  &c.  Svo,  Cloth,  $4  00. 

EDGEWORTH'S  (Miss)  NOVELS.   With  Engravings.    10  vols.,  12mo,  Cloth,  $15  00. 
GROTE'S  HISTORY  OP  GREECE.    12  vols.,  12mo,  Cloth,  $18  00. 

HELPS'S  SPANISH  CONQUEST.  The  Spanish  Conquest  in  America,  and  its  Rela- 
tion to  the  History  of  Slavery  and  to  the  Government  of  Colonies.  By  ARTHUR 
HELPS.  4  vols.,  12mo,  Cloth,  $6  00. 

BALE'S  (MBS.)  WOMAN'S  RECORD.  Woman's  Record :  or,  Biographical  Sketches 
of  all  Distinguished  Women,  from  the  Creation  to  the  Present  Time.  Arranged 
in  Four  Eras,  with  Selections  from  Female  Writers  of  each  Era.  By  Mrs.  SARAH 
JOSEPIIA  HALE.  Illustrated  with  more  than  200  Portraits.  8vo,  Cloth,  $5  00. 

HALL'S  ARCTIC  RESEARCHES.  Arctic  Researches  and  Life  among  the  Esqnl 
maux:  being  the  Narrative  of  an  Expedition  in  Search  of  Sir  John  Franklin,  in 
the  Years  18UO,  1861,  and  1862.  By  CHARLES  FRANCIS  HALL.  With  Maps  and  100 
Illustrations.  The  Illustrations  are  from  Original  Drawings  by  Charles  Parsons, 
Henry  L.  Stephens,  Solomon  Eytinge,  W.  S.  L.  Jewett,  and  Granville  Perkins, 
after  Sketches  by  Captain  Hall.  8vo,  Cloth,  $5  00. 

HALLAM'S  CONSTITUTIONAL  HISTORY  OF  ENGLAND,  from  the  Accession  of 
Henry  VII.  to  the  Death  of  George  II.  8vo,  Cloth,  $2  00. 

HALLAM'S  LITERATURE.  Introduction  to  the  Literature  of  Europe  during  the 
Fifteenth,  Sixteenth,  and  Seventeenth  Centuries.  By  HENRY  UALLAM.  2  vols., 

8vo,  Cloth,  $4  00. 

HALLAM'S  MIDDLE  AGES.  State  of  Europe  during  the  Middle  Ages.  By  HENRY 
HALI.AM.  8vo,  Cloth,  $2  00. 

HILDRETH'S  HISTORY  OF  THE  UNITED  STATES.  FIBST  SERIES  :  From  the 
First  Settlement  of  the  Country  to  the  Adoption  of  the  Federal  Constitution. 
SECOND  SERIES:  From  the  Adoption  of  the  Federal  Constitution  to  the  End  of 
the  Sixteenth  Congress.  6  vols.,  Svo,  Cloth,  $18  00. 

HUME'S  HISTORY  OF  ENGLAND.  History  of  England,  from  the  Invasion  of  Ju- 
lius Csesar  to  the  Abdication  of  James  II.,  16S8.  By  DAVID  HUME.  A  new  Edi- 
tion, with  the  Author's  last  Corrections  and  Improvement*  To  which  is  Prefix- 
ed a  short  Account  of  his  Life,  written  by  Himself.  With  a  Portrait  of  the  Au- 
thor. 6  vols.,  12mo,  Cloth,  $9  00. 

JAY'S  WORKS.  Complete  Works  of  Rev.  William  Jay:  comprising  his  Sermons, 
Family  Discourses,  Morning  and  Evening  Exercises  for  every  Day  in  the  lear, 
Family  Prayers,  &C,  Author's  enlarged  Edition,  revised.  3  vols.,  Svo,  Cloth, 
$600. 

JEFFERSON'S  DOMESTIC  LIFE.  The  Domestic  Life  of  Thomas  Jefferson  :  com- 
piled from  Family  Letters  and  Reminiscences  by  his  Great-Granddaiighter, 
SARAH  N  RANDOLPH.  With  Illustrations.  Crown  Svo,  Illuminated  Cloth,  Bev- 
eled Edges,  $2  50. 

JOHNSON'S  COMPLETE  WORKS.  The  Works  of  Samnel  Johnson,  LL.D.  With 
an  Essay  on  his  Life  and  Genius,  by  ARTHUR  MURPUY,  Esq.  Portrait  of  Johngon. 
2  vols.,  Svo,  Cloth,  $4  00. 

KINGLAKE'S  CRIMEAN  WAR.  The  Invasion  of  the  Crimea,  and  an  Account  of 
its  Progress  down  to  the  Death  of  Lord  Raglan.  By  AI.KXANI,«B  \V  11.1.1  AM  KINO- 
LAKE.  With  Maps  and  Plans.  Two  Vols.  ready.  12mo,  Cloth,  $2  00  per  voL 

KiNGSLEY'S  WEST  INDIES.  At  Last:  A  Christmas  in  the  West  Indiee.  By 
CII/.RI.KS  KiNOSi.r.v.  Illustrated.  l'2mo,  Cloth,  $1  50. 


6     Harper  6^  Brothers'  Valuable  and  Interesting  Works. 

KRUMMACHER'S  DAVID,  KING  OF  ISRAEL.  David,  the  King  of  Israel:  a  For- 
trait  drawu  from  Bible  History  and  the  Book  of  Psalms.  By  FREDERICK  WILLIAM 
KRUMMAODER,  D.D.,  Author  of  "Elijah  the  Tishbite,"  &c.  Translated  under  the 
express  Sanction  of  the  Author  by  the  Rev.  M.  G.  EASTON,  M.A.  With  a  Letter 
from  Dr.  Krummacher  to  his  American  Readers,  and  a  Portrait.  12mo,  Cloth, 
$1  75. 

LAMB'S  COMPLETE  WORKS.  The  Works  of  Charles  Lamb.  Comprising  his  Let- 
ters, Poems,  Essays  of  Elia,  Essays  upon  Shakspeare,  Hogarth,  &c.,  and  a  Sketch 
of  his  Life,  with  the  Final  Memorials,  by  T.  NOON  TALFOBRK.  Portrait  2  vols., 
12mo,  Cloth,  $3  00. 

LIVINGSTONE'S  SOUTH  AFRICA.  Missionary  Travels  and  Researches  in  Sonth 
Africa:  including  a  Sketch  of  Sixteen  Years'  Residence  in  the  Interior  of  Africa, 
and  a  Journey  from  the  Cape  of  Good  Hope  to  Loando  on  the  West  Coast ;  thence 
across  the  Continent,  down  the  River  Zambesi,  to  the  Eastern  Ocean.  By  DAVID 
LIVINGSTONE,  LL.D.,  D.C.L.  With  Portrait,  Maps  by  Arrowsmith,  and  numerous 
Illustrations.  Svo,  Cloth,  $4  50. 

LIVINGSTONES'  ZAMBESI.  Narrative  of  an  Expedition  to  the  Zambesi  and  its 
Tributaries,  and  of  the  Discovery  of  the  Lakes  Shirwa  and  Nyassa.  1858-1864. 
By  DAVID  and  CUARLEB  LIVINGSTONE.  With  Map  and  Illustrations.  Svo,  Cloth, 
$500. 

M'CLINTOCK  &  STRONG'S  CYCLOPEDIA.  Cyclopedia  of  Biblical,  Theological, 
and  Ecclesiastical  Literature.  Prepared  by  the  Rev.  JOIIN  M'CLINTOCK,  D.D., 
and  JAMES  STRONG,  S.T.D.  5  vols.  now  ready.  Royal  Svo.  Price  per  vol.,  Cloth, 
$5  00 ;  Sheep,  $6  00 ;  Half  Morocco,  $S  00. 

MARCY'S  ARMY  LIFE  ON  THE  BORDER.  Thirty  Years  of  Army  Life  on  the 
Border.  Comprising  Descriptions  of  the  Indian  Nomads  of  the  Plains ;  Explo- 
rations of  New  Territory;  a  Trip  across  the  Rocky  Mountains  in  the  Winter; 
Descriptions  of  the  Habits  of  Different  Animals  found  in  the  West,  and  the  Meth- 
ods of  Hunting  them  ;  with  Incidents  in  the  Life  of  Different  Frontier  Men,  &c., 
&c.  By  Brevet  Brigadier-General  R.  B.  MARCY,  U.S.A.,  Author  of  "  The  Prairie 
Traveller."  With  numerous  Illustrations.  Svo,  Cloth,  Beveled  Edges,  $3  00. 

MACAULAY'S  HISTORY  OF  ENGLAND.  The  History  of  England  from  the  Ac- 
cession of  James  II.  By  THOMAS  BAHINGTON  MAOAULAY.  With  an  Original  Por- 
trait of  the  Author.  5  vols.,  Svo,  Cloth,  $10  00 ;  12mo,  Cloth,  $7  50. 

MOSHEIM'S  ECCLESIASTICAL  HISTORY,  Ancient  and  Modern  ;  in  which  the 
Rise,  Progress,  and  Variation  of  Church  Power  are  considered  in  their  Connec- 
tion with  the  State  of  Learning  and  Philosophy,  and  the  Political  History  of  Eu- 
rope during  that  Period.  Translated,  with  Notes,  &c.,  by  A.  MACLAINE,  D.D. 
A  new  Edition,  continued  to  1826,  by  C.  COOTE,  LL.D.  2  vols.,  Svo,  Cloth,  $4  00. 

NEVTUS'S  CHINA.  China  and  the  Chinese:  a  General  Description  of  the  Country 
and  its  Inhabitants;  its  Civilization  and  Form  of  Government ;  its  Religious  and 
Social  Institutions ;  its  Intercourse  with  other  Nations ;  and  its  Present  Condition 
and  Prospects.  By  the  Rev.  JOHN  L.  NEVIUS,  Ten  Years  a  Missionary  in  China. 
With  a  Map  and  Illustrations.  12mo,  Cloth,  $1  75. 

THE  DESERT  OF  THE  EXODUS.  Journeys  on  Foot  in  the  Wilderness  of  the 
Forty  Years'  Wanderings ;  undertaken  in  connection  with  the  Ordnance  Survey 
of  Sinai  and  the  Palestine  Exploration  Fund.  By  E.  H.  PALMER,  M.A.,  Lord 
Almoner's  Professor  of  Arabic,  and  Fellow  of  St.  John's  College,  Cambridge. 
With  Maps  and  numerous  Illustrations  from  Photographs  and  Drawings  taken 
on  the  spot  by  the  Sinai  Survey  Expedition  and  C.  F.  Tyrwhitt  Drake.  Crown 
8vo,  Cloth,  $3  00. 

OLIPHANT'S  CHINA  AND  JAPAN.  Narrative  of  the  Earl  of  Elgin's  Mission  to 
China  and  Japan,  in  the  Years  1857,  '58,  '59.  By  LATTRENCB  OLIPHANT,  Private 
Secretary  to  Lord  Elgin.  Illustrations.  Svo,  Cloth,  $3  50. 

OLIPHANT'S  (MRS.)  LIFE  OF  EDWARD  IRVING.  The  Life  of  Edward  Irving, 
Minister  of  the  National  Scotch  Church,  London.  Illustrated  by  his  Journals  and 
Correspondence.  By  Mrs.  OLIPHANT.  Portrait.  Svo,  Cloth,  $3  50. 

RAWLINSON'S  MANUAL  OF  ANCIENT  HISTORY.  A  Manual  of  Ancient  His- 
tory, from  the  Earliest  Times  to  the  Fall  of  the  Western  Empire.  Comprising 
the  History  of  Chaldsea,  Assyria,  Media,  Babylonia,  Lydia,  Phoenicia,  Syria,  Ju- 
dfea  Eo-ypt,  Carthage,  Persia,  Greece.  Macedonia,  Parthia,  and  Rome.  By 
GKORGK^RAWLINSON,  M.A.,  Camden  Professor  of  Ancient  History  in  the  Univer- 
sity of  Oxford.  12mo,  Cloth,  $2  50. 


Harper  6-  Brothers'  Valuable  and  Interesting  Works.      7 

RECLUS'S  THE  EARTH.  The  Earth :  a  Descriptive  History  of  the  Phenomena  and 
Life  of  the  Globe.  By  ELISEE  REOLUS.  Translated  by  the  late  B.  B.  Woodward, 
and  Edited  by  Henry  Woodward.  With  234  Maps  and  Illustrations,  and  23  Page 
Maps  printed  in  Colors.  Svo,  Cloth,  $6  00. 

RECLUS'S  OCEAN.  The  Ocean,  Atmosphere,  and  Life.  Being  the  Second  Series 
of  a  Descriptive  History  of  the  Life  of  the  Globe.  By  ELISI£K  RKCLCB.  Profusely 
Illustrated  with  250  Maps  or  Figures,  and  27  Maps  printed  in  Colors.  Svo,  Cloth, 
$6  00. 

SHAKSPEARE.  The  Dramatic  Works  of  William  Shakspeare,  with  the  Corrections 
and  Illustrations  of  Dr.  JOHNSON,  G.  STEEVENS,  and  others.  Revised  by  ISAAC 
REED.  Engravings.  6  vols.,  Royal  12mo,  Cloth,  $9  00. 

SMILES'S  LIFE  OF  THE  STEPHENSONS.  The  Life  of  George  Stephenson,  and 
of  his  Son,  Robert  Siephenson ;  comprising,  also,  a  History  of  the  Invention  and 
Introduction  of  the  Railway  Locomotive.  By  SAMUEL  SMILES,  Author  of  "Self- 
Help,"  <fcc.  With  Steel  Portraits  and  numerous  Illustrations.  8vo,  Cloth,  $3  00. 

SMILES'S  HISTORY  OF  THE  HUGUENOTS.  The  Huguenots :  their  Settlements, 
Churches,  arid  Industries  in  England  and  Ireland.  By  SAMUEL  SMILES.  With  an 
Appendix  relating  to  the  Huguenots  in  America.  Crown  Svo,  Cloth,  $1  75. 

SPEKE'S  AFRICA.    Journal  of  the  Discovery  of  the  Source  of  the  Nile.    By  Cap- 


tain JOHN  BANNING  SPEKE,  Captain  H.  M.  Indian  Army,  Fellow  and  Gold  Med- 
alist of  the  Royal  Geographical  Society,  Hon.  Corresponding  Member  and  Gold 
Medalist  of  the  French  Geographical  Society,  &c.  With  Maps  and  Portraits  and 


numerous  Illustrations,  chiefly  from  Drawings  by  Captain  GEANT.    Svo,  Cloth, 
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